The United States on Wednesday vetoed a United Nations Security Council resolution calling for an immediate and unconditional cease-fire between Israel and Hamas in the Gaza Strip, where fighting has raged for more than 13 months and a humanitarian crisis is intensifying.

Underlining Washington’s diplomatic isolation on the issue, the United States cast the sole vote against the resolution, with the 14 other Council members voting in favor.

The United States said it vetoed the resolution, the fifth the Council has taken up, because it did not make the cease-fire contingent on the release of the hostages still being held in Gaza. The resolution does call for the release of all hostages, but the wording suggests that their release would come only after a cease-fire was implemented.

The impasse at the United Nations appeared in contrast to cease-fire talks in Lebanon, where a top U.S. envoy, Amos Hochstein, said on Wednesday that there had been “additional progress” in efforts to halt the fighting between Israel and Hezbollah, the Lebanese militant group allied with Hamas. Mr. Hochstein, saying he hoped “to try to bring this to a close if we can,” traveled to Israel on Wednesday evening.

The leader of Hezbollah said Wednesday that the war’s end was now in the hands of Israeli leaders.

The U.S. veto on Wednesday was the fourth time the United States has blocked an effort by the Council to demand a cease-fire in Gaza since last year, when Hamas led an attack on Israel that killed about 1,200 people and took more than 200 others hostage. More than 40,000 people have been killed in Gaza since then, according to the local health authorities, and the territory faces the risk of famine, experts say.

In speech after speech at the Council on Wednesday, the 14 diplomats who voted for the resolution said the suffering, death, displacement and starvation unfolding in Gaza was catastrophic and unacceptable. Ending the fighting, they said, was a necessary first step to release the hostages, about 100 of whom remain in Gaza, and to save civilians.

In the eyes of many nations, the Biden administration’s legacy will be colored by its staunch defense of Israel.

“This will sadly further weaken Biden’s reputation in the U.N. bubble,” said Richard Gowan, the U.N. director of the International Crisis Group. “He was once seen as a welcome replacement for Trump. But he leaves office with low marks among most U.N. diplomats who think he has been egregiously obstructive on Gaza.”

Although Security Council resolutions are considered international law, the Council lacks enforcement powers. It could impose punitive measures, such as sanctions, but that would also require member states to agree.

Washington, while working with Qatar and Egypt for months for a cease-fire deal and the release of hostages, has backed Israel’s position that one cannot come before the other.

“We could not support an unconditional cease-fire that failed to release the hostages,” said Robert A. Wood, an American ambassador to the United Nations. “These two urgent goals are inextricably linked. This resolution abandoned that necessity.”

After the vote, Israel’s ambassador, Danny Danon, thanked the United States and blamed Hamas for the suffering of Palestinian civilians. “The resolution brought before this chamber was not a path to peace,” he said. “It was a road map to more terror, more suffering, and more bloodshed.”

The two-page resolution called for an immediate and unconditional cease-fire; the withdrawal of Israeli troops from Gaza; increased and unhindered delivery of humanitarian aid; and for all parties to enable the battered U.N. agency that helps Palestinians, known as UNRWA, to carry out its work in Gaza and the West Bank.

“A cease-fire will allow to save lives, all lives,” said Majed Bamya, the deputy Palestinian ambassador to the Council. “A cease-fire doesn’t resolve everything, but it is the first step toward resolving anything.”

The resolution was put forth by the 10 nonpermanent Council members: Algeria, Ecuador, Guyana, Japan, Malta, Mozambique, South Korea, Sierra Leone, Slovenia and Switzerland.

“It is a sad day for the Security Council, for the United Nations and for the international community,” Algeria’s ambassador, Amar Bendjama, said after the vote.

The draft resolution was negotiated for weeks, said Guyana’s ambassador, Carolyn Rodrigues-Birkett. She said the Council needed to respond to concerns “over the catastrophic humanitarian situation in Gaza.”

The Council, where the United States is rivaled by Russia and China, has struggled to speak in one voice and play an effective role in mediating the wars in Ukraine and the Middle East. Russia and China vetoed an American resolution in March that called for “an immediate and sustained cease-fire” in Gaza.

Aid groups condemned the U.S. veto on Wednesday.

“It is shameful that the United States has once again used the veto to block consensus on a lifesaving cease-fire, while they continue to approve deadly arms transfers to fuel the violence and humanitarian catastrophe,” said Brenda Moyfa, head of Oxfam’s New York office.

The United States’ support for Israel, and the resulting deadlock over Gaza in the Council, has also generated frustration from some of America’s closest allies, the permanent members Britain and France.

“We deplore today’s failure of the U.N. Security Council,” said France’s ambassador, Nicolas de Rivière. He said the resolution sent the right messages: the immediate cease-fire and the immediate release of hostages. “There is no credible justification to object to that.”

Speaking to reporters, Mr. de Rivière also said French and American officials were discussing the war in Lebanon, and specifically proposals to monitor southern Lebanon during a cease-fire.

Mr. de Rivière said the Lebanese army needed to move troops to southern Lebanon as part of a cease-fire, which would be based on a 2006 Security Council resolution that ended Israel’s last war in Lebanon.

On Wednesday in Beirut, Mr. Hochstein, the U.S. envoy, met for a second straight day with Nabih Berri, the Hezbollah-allied speaker of Lebanon’s Parliament, who has emerged as an important figure in negotiations. During many rounds of diplomacy over the past year, Mr. Hochstein has seldom stayed overnight in Lebanon, so his extended visit raised hopes for progress in talks.

Mr. Hochstein was set to meet with Prime Minister Benjamin Netanyahu of Israel on Thursday, said a spokesman, Omer Dostri.

In a televised address during Mr. Hochstein’s visit, Hezbollah’s new leader, Naim Qassem, said Hezbollah had responded to the U.S. cease-fire proposal. A truce now depended on Israel’s response and Mr. Netanyahu’s “seriousness,” he said.

“Any cease-fire negotiations must adhere to two demands: the total cessation of hostilities by Israel, and the preservation of Lebanese sovereignty,” he said.

If negotiations broke down, he warned, Hezbollah was prepared for a “long war.”

Israel’s conflict with Hezbollah, which is backed by Iran, escalated in September and has resulted in a humanitarian crisis, killing more than 3,500 people in Lebanon and displacing almost a quarter of the population. It is now the bloodiest conflict there since Lebanon’s civil war, which lasted from 1975 to 1990.

Reporting was contributed by Aaron Boxerman in Jerusalem, Johnatan Reiss in Tel Aviv, Dayana Iwaza in Beirut, and Matthew Mpoke Bigg in London.

The Biden administration has approved supplying Ukraine with American anti-personnel mines to bolster defenses against Russian attacks as Ukrainian front lines in the country’s east have buckled, Defense Secretary Lloyd Austin said on Wednesday.

The decision is the latest in a series of moves by Russia and the United States related to the war in Ukraine that have escalated tensions between the two.

The White House recently granted permission to Ukraine to fire longer-range American missiles at targets in Russia, which the Ukrainians did for the first time on Tuesday. Moscow in response formalized a new doctrine lowering the threshold for when it would use nuclear weapons.

Mr. Austin said the U.S. decision was prompted by Russia’s increasing reliance on foot soldiers to lead their assaults, instead of armored vehicles. Mr. Austin, speaking to reporters while traveling in Laos, said the shift in policy follows changing tactics by the Russians. Because of that, Ukraine has “a need for things that can help slow down that effort on the part of the Russians,” Mr. Austin said.

“They’ve asked for these, and so I think it’s a good idea,” Mr. Austin said.

The mines that will be provided to Ukraine are called “nonpersistent,” and are designed to self-destruct after a period of time to reduce the long-term threat to noncombatants.

The move is also noteworthy because it is part of a series of actions taken in the waning weeks of the Biden presidency to bolster Ukraine. President Biden in the past has sought to calibrate American help for Ukraine against his own concern about crossing Russian “red lines” that could lead to direct conflict between Washington and Moscow.

But since the Nov. 5 election that will bring former President Donald J. Trump back to the White House, Biden administration officials have said the potential benefits of the actions outweigh the escalation risks.

The announcement came on a day of increased anxiety in Kyiv, Ukraine’s capital, and across the country. The United States closed its embassy in Kyiv, warning of a “significant air attack,” as Ukraine and the West brace for more intensive assaults by the Russians.

Mines in general have been devastatingly effective in the war in Ukraine, and Russia has made extensive use of them. The mines are planted by hand but can also be scattered remotely with rockets or drones behind opponents’ lines, to catch soldiers as they move to and from positions, a tactic that can assist an offensive.

Land mines, however, have been most effective in defense. A broad belt of dense minefields in southern Ukraine stymied a Ukrainian counteroffensive in the summer of 2023 and gravely wounded a large but undisclosed number of Ukrainian soldiers.

Most anti-personnel mines are small explosives about the size of a hockey puck that are triggered by the pressure of a footstep.

The Biden administration’s decision came despite widespread condemnation of mines by rights groups that cite their toll on civilians, which can stretch for years or decades after conflicts end as the locations of minefields are left unmarked or forgotten. Ukraine is already the most heavily mined country in the world, according to the United Nations.

Most countries, but not the United States and Russia, are signatories of a convention banning the use or stockpiling of land mines, the 1997 Ottawa Treaty. Ukraine is a signatory to the agreement.

In a report released in October, the United Nations said that since 2022, 407 Ukrainian civilians have died and 944 were wounded by mines and unexploded ordnance.

An investigation by the rights group Human Rights Watch in 2023 pointed to the use of rocket-dispersed land mines by Ukrainian troops near the eastern town of Izium in 2022. Ukraine’s foreign ministry said it would investigate the allegation. As the United States is not a signatory, it is not obliged to refrain from supplying land mines to other countries.

A spokeswoman for the Ukrainian ministry of defense did not respond to a query on the decision to transfer American land mines to Ukraine.

Russia has seeded mines throughout vast swaths of Ukraine since 2014 as front lines have swayed over forests, farm fields and villages. It has also set many so-called victim-activated booby traps, such as explosives rigged to detonate when a car door is opened, a category of weapon also prohibited in the mine ban treaty.

President Biden has authorized the delivery of U.S.-made anti-personnel mines to Ukraine, heeding Kyiv’s pleas for military aid to bolster its defenses against the Russian invasion.

For centuries, militaries around the world have relied on land mines as a lethal and cost-effective way to defend territory. Once in place, many of them can stay armed and deadly indefinitely. But for that reason, human rights groups say they pose a grave and indiscriminate threat to civilians, for years or decades after a conflict has ended.

U.S. officials on Wednesday said that they were addressing those concerns by only providing Ukraine with anti-personnel mines that self-destruct after a set amount of time.

What are anti-personnel mines?

Anti-personnel mines are small, explosive weapons designed to detonate when a person steps on them, or comes close to them, according to Mine Action Review, a nonprofit that monitors the use of these weapons. Militaries typically deploy mines as defensive weapons, to prevent enemy forces from approaching or overtaking a certain area.

There are many different kinds of anti-personnel mines, and varied ways to distribute them. Some are shaped like hockey pucks, ranging from 3 to 16 inches in diameter, while others are shaped like a cylinder or cone.

They are designed to inflict different levels of injury. “Blast” mines, for instance, are designed to detonate with a range of force that could maim a victim or explode with enough force to kill someone, according to the United Nations. Other anti-personnel mines spray shrapnel into the air, sometimes bounding a few feet into the air before exploding.

Anti-personnel mines can be buried below the surface of the ground, or placed in a booby-trapped building. They can also sit on top of the ground; once activated by a short time delay, they launch a number of thin tripwires in different directions. When a person or an animal disturbs one of those wires, the mine explodes.

Modern land mines can be scattered by artillery shells, rockets, cluster bombs and launchers mounted onto helicopters or trucks.

Who is using anti-personnel mines?

Mines have been used since the American Civil War. They were used heavily during World War II, and American forces routinely employed them during the Korean and Vietnam wars. According to government records, the U.S. military last used anti-personnel mines widely during the Persian Gulf war in 1991.

The last reported use of anti-personnel mines by American forces came in 2002, when U.S. special operations forces in Afghanistan used a single hand grenade made from a repurposed anti-personnel mine, known as a pursuit deterrent munition.

Russian forces have used more than a dozen different types of anti-personnel mines in Ukraine, dating as far back as the invasion of Crimea in 2014, according to Human Rights Watch. Since the full-scale invasion of Ukraine in 2022, Russia has continued to deploy anti-personnel mines, with the weapons detected in 11 of 27 regions in Ukraine.

Last year, an investigation by Human Rights Watch found that Ukrainian soldiers had also fired artillery rockets containing anti-personnel land mines into a Russian-controlled area of eastern Ukraine.

Less than two months after Russia launched its invasion of Ukraine, the United States provided Ukraine with Claymores, which are anti-personnel mines placed above ground and detonated at the user’s discretion, making them allowable under international conventions if used correctly.

In September 2022 the Pentagon sent Ukraine artillery shells called RAAMS — for Remote Anti-Armor Mine System — that break open midair and dispense anti-tank mines.

Why are anti-personnel mines controversial?

Land mines can indiscriminately kill and maim noncombatants, years or even decades after the cessation of hostilities.

In 2022, land mines killed 1,661 people and injured 3,015, according to the International Campaign to Ban Landmines. Civilians made up 85 percent of those casualties, half of whom were children. Syria had the highest number of injuries and deaths, followed by Ukraine, Yemen and Myanmar.

Armies or groups of fighters that employ anti-personnel mines rarely map the areas they have mined. It is difficult for most people, especially children, to know where mines could be lurking.

Over the past 20 years, most countries have retired and destroyed their stockpiles of anti-personnel mines because of concerns about indiscriminate harm.

Older land mines are commonly known as “persistent” mines, meaning they can stay armed and lethal for many years. They use mechanical fuzes that enable the mine to explode as long as its internal mechanisms stay intact.

In the 1970s, the United States military developed a newer type of anti-personnel mine it calls “nonpersistent,” which incorporates electronics that allow the device to self-destruct after a preset amount of time. However, those safety mechanisms sometimes fail.

The lasting harm from land mines came to international attention in 1997, when Diana, the Princess of Wales, walked through a cleared minefield in Angola. Her campaign led to the 1997 Mine Ban Treaty, known formally as the Convention on the Prohibition of the Use, Stockpiling, Production and Transfer of Anti-Personnel Mines and on Their Destruction. To date, 164 countries have formally agreed to the ban.

Ukraine is a signatory to the treaty, but has been tight-lipped about its use of the weapons since Russia’s invasion. The United States and Russia have not signed the treaty.

In 2022, the Biden administration reversed a Trump administration policy that loosened the restrictions on the U.S. military’s use of anti-personnel mines. The authorization announced this week appears to contradict that stance.

Three key European nations and the United States have moved to censure Iran over its secretive nuclear program, hoping to shore up the credibility of the world’s nuclear watchdog before Donald J. Trump’s return to the White House.

The United States and the three European nations involved — Britain, France and Germany — put forward a resolution on Tuesday condemning Iran for its consistent refusal to answer questions from the International Atomic Energy Agency, the U.N.’s nuclear monitoring organization, about its nuclear enrichment program.

The resolution to censure may be voted on at the agency’s regular meeting of its board of governors in Vienna as soon as Thursday. If the measure passes, it could ultimately lead to retaliatory measures against Iran by the West.

The censure resolution followed the circulation of a report at the agency earlier this week detailing Iran’s expansion of its stockpile of enriched uranium that is close to weapons grade and its consistent efforts to block the IAEA from monitoring its progress.

The countries pushing for the censure are concerned that Iran’s continued recalcitrance presents a significant threat to the Nuclear Nonproliferation Treaty and undermines the credibility of the IAEA itself.

Analysts said that the European countries are also trying to signal that they want to be tough on Iran, wary that the American president-elect may be tempted to cut a separate deal with Iran that also undermines the role of the IAEA.

In a change from past Biden Administration policy, the United States has decided to support the resolution as well.

“We remain tightly coordinated with our E3 partners (France, Germany and Britain) in advance of the IAEA Board of Governors meeting, and we strongly support efforts to hold Iran accountable,” a U.S. State Department spokesman, Matthew Miller, told reporters on Tuesday in Washington.

Less than 1 percent of the uranium found in nature is the more radioactive form, known as U-235. Uranium enrichment raises that fraction, to 3 to 5 percent for nuclear power plants, and up to 20 percent for some kinds of research. An atomic bomb requires enrichment to 85 percent U-235 or more.

Iran has tried to avoid the condemnation by proposing to cap its production of 60 percent enriched uranium — close to weapons-grade and having no known civilian use — for the foreseeable future and expressing willingness to accept four new agency inspectors to replace the ones Iran banned from entering in September 2023. Experts say uranium at 60 percent can be raised to bomb-grade in a relatively short time.

But these are seen as more symbolic gestures aimed at avoiding censure and signaling a willingness to negotiate further with a new American administration on limits to Iran’s nuclear program, in return for the lifting of economic sanctions. If censured, however, Iran warns that it will retaliate and would be unlikely to comply with any proposal to cap enrichment.

Everyone is waiting to see what kind of Iran policy Mr. Trump will support once he is back in office, diplomats and analysts say.

“The Europeans want to show Trump that they are happy to work with him on a tough new joint policy on Iran, rather than Trump excluding them and doing his own deal,” said Ellie Geranmayeh, an expert on Iran with the European Council on Foreign Relations.

There is an assumption that as president again, Mr. Trump will try to increase economic pressure on Iran to try to reach a stricter deal than the 2015 agreement that was meant to restrain Iran’s nuclear program.

Mr. Trump criticized that deal as weak and pulled out of it in 2018, reimposing American sanctions on Iran. His policy of “maximum pressure” did not bring Iran to the table then, and neither he nor President Biden has been able to replace the 2015 agreement with a new one.

Instead Iran has steadily increased its stockpile of highly enriched uranium, enough for several bombs, and is closer to being able to build a nuclear weapon and deliver one on more sophisticated missiles. Iran insists its nuclear program is civilian and that it has no intention to build a nuclear weapon.

But diplomats and analysts say that Iran is now perceived to have been weakened by internal unrest, a bad economy, and Israel’s campaign against Iran and its allies since Hamas invaded Israel Oct. 7 of last year. There is a significant internal debate in Iran about whether to try to do a deal again or move more openly toward building a nuclear weapon, they say.

The diplomats and analysts believe that there is a good chance Iran would be open to direct negotiations on a new nuclear deal with Washington, along with the Europeans. But Iran would want such a deal to provide clear economic benefits and restrain the United States and Israel from attacking its nuclear and energy facilities.

European and Iranian officials met in September at the United Nations, after the death of Iran’s hard-line president and the election of a more moderate successor, to discuss de-escalation and re-engagement, with little obvious success.

But the Europeans were struck by the quiet meeting between Elon Musk and Iran’s U.N. ambassador on Nov. 11, first reported in The New York Times, as indicative of tentative early efforts to defuse tensions.

There is an extensive and relatively open debate inside Iran about the future of its nuclear program, said Ms. Geranmayeh and Cornelius Adebahr, who studies Iran for Carnegie Europe.

As they describe the debate, some voices say that Iran has a better chance negotiating with a Republican president who wants to avoid a war and controls Congress and that this may be a last chance to do so. The hope is that it would yield economic benefits and also help to calm dissent at home.

Others, many of them part of the powerful Islamic Revolutionary Guards Corps, argue that the West cannot be trusted and that the United States is a declining power. Israel, they also argue, has hit its regional allies so hard that Iran should move more openly to confront Israel and develop a nuclear deterrent.

“Superficially Iran looks weaker, with a prevalent internal atmosphere of unrest and the regime trying to find its footing, and the openness of the debate speaks to the uncertainty among policymakers in Iran,” Mr. Adebahr said. “But the Iranians don’t consider themselves weak and don’t appear to be in a great rush.”

Whatever happens to nuclear negotiations, the issue of compliance with the IAEA’s inspectors is a different matter. The Europeans and Washington consider that Iran’s consistent flouting of its obligations goes to the heart of preserving the nonproliferation treaty and preventing other countries in the Middle East and elsewhere from developing their own nuclear weapons.

The agency’s reports made clear that Iran has failed to resolve questions about the origins of undeclared nuclear material found earlier in Iran and had not restored monitoring equipment, including cameras, it removed in June 2022.

The resolution censoring Iran, if passed, would call for a comprehensive report on all the open questions about its nuclear program. It could also open the way for a return to all the international sanctions suspended under the 2015 deal, though the deadline for that is October 2025.

A top U.S. envoy said on Wednesday that he would travel to Israel after two days of progress in cease-fire talks in Lebanon, as the leader of the Lebanese militant group Hezbollah said an end to its war with Israel was now in the hands of Israeli leaders.

Amos Hochstein, the senior Biden administration envoy, said at a news conference in Beirut that there had been “additional progress” in his discussions with Lebanese officials, and that he would go to Israel later Wednesday “to try to bring this to a close if we can.”

Mr. Hochstein met for a second straight day with Nabih Berri, the Hezbollah-allied speaker of the Lebanese Parliament, who has emerged as the group’s point man in the U.S.-led negotiations. During repeated rounds of shuttle diplomacy over the past year, Mr. Hochstein has seldom stayed overnight in Lebanon, so his extended visit raised hopes that negotiations were moving ahead.

The office of Benjamin Netanyahu, the Israeli prime minister, confirmed that Mr. Hochstein was expected to arrive in Israel on Wednesday night for further talks. Omer Dostri, the prime minister’s spokesman, said Mr. Hochstein was set to meet with Mr. Netanyahu on Thursday.

In a televised address during Mr. Hochstein’s visit, Hezbollah’s new leader, Naim Qassem, said that Hezbollah had provided its response to the U.S. cease-fire proposal. He added that a truce now depended on Israel’s response and the “seriousness” of Mr. Netanyahu.

“Any cease-fire negotiations must adhere to two demands: the total cessation of hostilities by Israel, and the preservation of Lebanese sovereignty,” Mr. Qassem said.

If negotiations broke down, he warned, Hezbollah was prepared for a “long war.”

Israel’s conflict with Hezbollah, a group backed by Iran, escalated in September and has resulted in a humanitarian crisis, killing more than 3,500 people in Lebanon and displacing almost a quarter of the population. It is now the bloodiest conflict inside Lebanon since the country’s civil war, which lasted from 1975 to 1990.

The Biden administration dispatched Mr. Hochstein to the region in what amounted to a last-ditch effort to close a deal before President-elect Donald J. Trump takes office in January. Mr. Hochstein said on Wednesday that the Biden administration would coordinate with the incoming Trump administration over cease-fire talks in Lebanon, and that he did not believe the transfer of power in Washington would imperil those efforts.

“We are going to work with the incoming administration. We are already going to be discussing this with them,” he said.

Here are other developments:

• Strike in Syria: Dozens were killed in an Israeli airstrike in Palmyra, a city in central Syria, according to SANA, the Syrian state news agency. The Syrian Observatory of Human Rights, a group based in Britain that tracks conflicts in Syria, said 41 were killed in the strike, including 22 foreign citizens. The Israeli military declined to comment. Israeli officials have recently been more forthcoming about Israel’s attacks in Syria, stating that they were aimed at disrupting militant groups including Hezbollah.

• Israeli soldier killed: The Israeli military said a soldier was killed in combat in northern Gaza, the 800th Israeli soldier to die in the enclave since the war began in October 2023. It said another soldier had been wounded in the same incident on Tuesday. The Israeli news media reported that the two had been ambushed in a building in Beit Lahia, one of the towns in northern Gaza where Israel has mounted a weekslong offensive against Hamas militants.

• Hospital struggles: The director of one of the last functioning hospitals in northern Gaza, Kamal Adwan Hospital, said the facility was providing only limited care to dozens of patients amid the Israeli military offensive. The director, Hussam Abu Safyia, said in comments reported by Gaza’s civil defense service that the hospital lacked food, water and medical supplies. Israeli forces withdrew from the hospital in late October after a raid during which Palestinian health officials said nearly all of the medical workers were detained.

Aaron Boxerman contributed reporting from Jerusalem, Johnatan Reiss from Tel Aviv, Dayana Iwaza from Beirut and Matthew Mpoke Bigg from London.

A prominent Ugandan opposition figure who disappeared last week while in neighboring Kenya surfaced on Wednesday in a military court back home, where he was charged with security-related offenses, his wife and Ugandan officials said — the latest case to raise alarm amid a widening opposition crackdown in both East African nations.

Kizza Besigye, a former presidential candidate in Uganda, was “kidnapped” on Saturday while visiting the capital, Nairobi, for a book launch by a Kenyan politician, his wife, Winnie Byanyima, said on social media early on Wednesday. Ms. Byanyima did not elaborate on how Mr. Besigye was abducted or by whom. But she said he was remanded to a military jail in the Ugandan capital, Kampala, and did not have access to his family or lawyers.

“He is not a soldier. Why is he being held in a military jail?” said Ms. Byanyima, who is the executive director of U.N.AIDS, the United Nations program on H.I.V. and AIDS. She did not respond immediately to an attempt to reach her by email.

Hours later, Mr. Besigye, surrounded by security officers and holding his fingers in a V-sign, appeared at a military court in Kampala, according to footage broadcast on public and private television stations. He and an associate — Haji Obeid Lutale — were remanded to prison pending trial on charges including unlawful possession of firearms and ammunition, according to a charge sheet seen by The New York Times.

Authorities alleged that Mr. Besigye, Mr. Lutale and others still at large have, over the past year, held meetings in Switzerland, Greece and Kenya “aimed at soliciting for logistical support and identifying military targets in Uganda with intent to prejudice the security of the Defense Forces.”

Both Mr. Besigye and Mr. Lutale denied the charges.

Uganda’s government has not commented on the situation. Felix Kulayigye, a spokesman for the Ugandan army, did not respond to questions about how Mr. Besigye arrived in Uganda or appeared in military court. But, “I believe Kenya is a sovereign country and they can answer your questions,” he said in a text message.

Korir Sing’Oei, the principal secretary of Kenya’s foreign ministry, did not respond to a request for comment.

Mr. Besigye, who is also a physician, has in the past been arrested and assaulted by security officers, placed under house arrest to prevent him holding political rallies, and accused of treason and rape. He was acquitted of rape, and the treason charges were later quashed.

Other Ugandans who have challenged the decades-long rule of President Yoweri Museveni have faced similar fates. Mr. Museveni, a key Western ally, has governed the East African nation with an iron fist for almost four decades by muzzling the press and jailing and torturing detractors while winning elections marred by allegations of fraud.

Bobi Wine, a pop star who has become Mr. Museveni’s foremost challenger, called Mr. Besigye’s latest arrest “most unfortunate” and “of greatest concern.”

Mr. Wine, whose real name is Robert Kyagulanyi, has previously been beaten and tear-gassed and was shot in the leg by the police in September. Mr. Wine has also accused the president, his son and other top government officials of committing crimes against humanity and has filed, along with others, a case against them in the International Criminal Court.

On Wednesday, both Kenyan and Ugandan opposition officials decried the mysterious way in which Mr. Besigye was removed from Kenya — which has seen a wave of abductions in recent months. These include the kidnap and torture of activists and protesters who have been agitating against the government of President William Ruto.

“Kenya must decide whether we want to be a constitutional democracy governed by the primacy of the Bill of Rights and tenets of justice, or a tyranny,” James Orengo, the governor of Siaya County and a member of the opposition Orange Democratic Movement, said on social media.

In October, the United Nations refugee agency said it was “deeply concerned” that Kenya had sent back four Turkish refugees who had been abducted by masked men at gunpoint. In July, three dozen Ugandan opposition members who had traveled to Kenya to participate in a governance course were deported. They were charged with terrorism-related offenses once they arrived home.

“Ruto’s government is not only perfecting illegal abductions of its citizens, but it is facilitating and participating in international abductions in total disregard of Kenyan and international laws on due process and substantive justice,” Waikwa Wanyoike, a Kenyan constitutional lawyer, said in an interview.

Musinguzi Blanshe contributed reporting from Kampala, Uganda.

The United States Embassy in Kyiv issued an urgent warning on Wednesday morning that Russia might launch “a significant air attack,” closing the embassy and telling employees to shelter in place.

Air-raid alerts are a daily fact of life in Ukraine and the capital often comes under drone and missile attacks, but the U.S. embassy rarely issues such a specific alert or temporarily shuts down. At least two other Western embassies — Greece and Italy — announced that they too would close for the day after the U.S. warning.

The warning came one day after Ukraine’s military used American-made ballistic missiles to strike into Russian territory for the first time, after receiving long-sought authorization from President Biden to do so. The Kremlin had long warned that such strikes would be treated as an escalation, and on Tuesday vowed to respond.

“We will be taking this as a qualitatively new phase of the Western war against Russia,” Russia’s foreign minister, Sergei V. Lavrov, said at a news conference on Tuesday. “And we will react accordingly.”

In its message on Wednesday, the U.S. Embassy said it had “received specific information” about a potential attack, but did not offer details. It urged Americans to pay special attention to air-raid alerts.

Just before 2 p.m., the Ukrainian authorities warned about a potential ballistic missile attack and urged people in Kyiv to seek shelter.

As alarm spread and Kyiv residents hurried to shelters, Ukraine’s military intelligence agency released a statement suggesting that Russia was seeking to stoke panic by spreading rumors about the scale of a potential attack.

Russia has launched a number of deadly strikes on Ukraine this week, including an hourslong nationwide assault on Sunday that killed at least nine people. A rocket strike later that day on a residential building in the city of Sumy, near the Russian border, killed 10 people. Then an attack in the port city of Odesa killed 11 more people, and another on Monday night in the Sumy region killed 11. Scores were injured in the attacks.

Overnight and into Wednesday morning, air-raid alerts warned of incoming attack drones for most of the country. Ukraine’s air force said that it had destroyed 56 drones before noon.

One explosion rang out in Kyiv just before 8 a.m. when air-defense teams intercepted a drone, according to Ukrainian officials, who said falling debris had started a fire at a multistory residential building. There was no immediate information on casualties.

Such drone attacks have become increasingly common in recent weeks. During 1,000 days of war, Russia has targeted the capital with more than 2,500 missiles and drones, according to data collected by the city’s military administration. Around half of the attacks took place this year.

Since the war began, there have been about 1,370 alerts in Kyiv, according to city officials. Those have lasted more than 1,550 hours in total — meaning that if residents spent every hour of every alert in a shelter, they would have spent more than two months in bunkers.

Many people seek shelter in subways, basements and underground facilities like parking garages when the air-raid warnings wail.

But there is often little warning when ballistic missiles, which travel at several times the speed of sound, are fired at the capital. The time between launch and impact can be minutes.

And many large-scale Russian attacks — like the one on Sunday, which targeted Ukraine’s power grid — feature a combination of drones, cruises and ballistic missiles aimed to overwhelm Ukrainian air defenses.

Both Moscow and Kyiv appear to be stepping up their attacks ahead of President-elect Donald J. Trump’s inauguration in January.

Mr. Trump has said he wants to bring a swift end to the war in Ukraine but has not said how, leading to speculation over whether he will maintain the same level of robust military support provided to Ukraine under the Biden administration.

President Volodymyr Zelensky of Ukraine has said he believes that the only way to force Moscow into peace negotiations is by showing strength and shoring up Ukraine’s position on the battlefield — with the help of its allies. He drove that point home again in an interview with Fox News that was broadcast Tuesday evening.

As long as Europe, the United States and the people of Ukraine remain united, he said, they could force President Vladimir V. Putin of Russia to accept a just and lasting peace.

“Putin is weaker than the United States of America,” Mr. Zelensky said. And Mr. Trump, he added, “is much stronger than Putin.”

President Biden’s decision to allow the Ukrainians to use the American-made ballistic missiles to strike inside Russia was a major change in U.S. policy — just two months before Mr. Trump heads to the White House.

Ukraine had been pleading for permission to use them for months, saying it needed longer-range capabilities to hit the Russian war machine. The weapon, known as the Army Tactical Missile System, or ATACMS, can reach further into Russia than any other Ukrainian missile.

But Ukraine has also been developing its own long-range weapons. Mr. Zelensky said on Tuesday that the country would produce at least 30,000 long-range drones next year.

On Wednesday, Ukraine’s military said it had used drones to target military installations in several regions of Russia overnight, including in the Novgorod region near the village of Kotovo, more than 400 miles from the Ukrainian border.

Russia’s Ministry of Defense said it had shot down 44 Ukrainian drones overnight, including 20 over the Novgorod region. Neither side’s claims could be independently verified.

Residents of Kyiv remained on edge even after the afternoon warning about a potential ballistic missile strike was lifted.

Olga Zasiadvovk, 28, said that as a Ukrainian living in Kyiv who has endured countless bombardments, it was only natural that “a constant sense of danger” created anxiety.

With rumors swirling that a particularly large attack might be imminent, she said, she was nervous but trying to control her emotions.

“Understanding that I don’t know when this will end, I’m learning to manage it,” she said.

Yelyzaveta Tolubko, 35, said she had discussed the same rumors with friends in a group chat on Wednesday.

“Two of them are in a panic,” she said. “One canceled her dentist appointment, and the other started messaging our clients who had fittings scheduled today to check if they’re still coming because she doesn’t want to go. She’s scared. The third is calm, but overall, the mood has soured.”

“We’re not exactly cheerful every day, but today there’s this added tension,” she added.

Liubov Sholudko contributed reporting.

On Wednesday, the Vatican made public new rules to simplify papal funerals, reflecting Pope Francis’s yearslong commitment to shedding some of the Church’s old rituals and papal pageantry.

The changes — which include reducing the number of coffins in which the pope’s body is laid to one from three, and allowing the pope to be buried in a church other than St. Peter’s Basilica — are another testament to Francis’s longtime embrace of a more down-to-earth, informal style that has become a trademark of his papacy.

The new ritual “also needed to emphasize even more that the funeral of the Roman pontiff is that of a pastor and disciple of Christ,” Msgr. Diego Ravelli, the Vatican’s master of pontifical liturgical ceremonies, said in a statement. “And not of a powerful person of this world.”

From the beginning of his papacy, Francis has made a point of using symbols to break from the formality and pomp that has long marked the Roman Catholic Church. He has worn Casio watches and used modest cars, and instead of living in the apostolic palace, he has lived in Casa Santa Marta, a residence next to St. Peter’s where he takes his meals in a communal cafeteria.

With the new rules, experts say, Francis has taken another step to seal it into his legacy.

“Today it’s more like the funeral of a diocesan bishop rather than of a Roman emperor,” said Massimo Faggioli, a professor of theology at Villanova University,

Francis approved the liturgical book containing the new rules in April. It updates an edition that had been approved by Pope John Paul II and published in 2000. Francis had previously expressed his intention to update the funeral, telling a Mexican broadcaster last year that he wanted to be buried in Santa Maria Maggiore Basilica in Rome, not in St. Peter’s like most of his predecessors. Francis visited Santa Maria Maggiore the day after his election and goes there before and after international journeys to pray in front of an icon of the Virgin Mary, the Salus Populi Romani (salvation of the people of Rome).

According to the new rules, the body of the pope will lie for the viewing directly in the coffin, not on an elevated bier as in the past. The pope’s cypress coffin will also no longer be inserted into a second coffin made of lead, and then in a third one made of oak or another wood.

The funeral of Pope Benedict XVI last year was held according to the old rituals. Francis, 88, has suffered a string of health problems and surgeries in recent years, but he mostly appears to be in good form.

A small band of Ukrainian soldiers was trapped. They were holding the line on the battlefield, but Russian forces had managed to creep in behind their trench and encircle them.

“Even if the position holds, supplies — ammunition, provisions — eventually run out,” Capt. Viacheslav, the 30-year-old commander of an elite drone unit, said last week as he monitored events from an outpost a few miles away in eastern Ukraine. “Any vehicle attempting to reach these positions will be ambushed.”

“We are always getting stuck in these kinds of tough situations,” he said.

As the war in Ukraine enters its fourth winter and the first snowfall blankets cratered fields strewed with bodies, the situations are only growing tougher for Ukrainian forces.

Gen. Oleksandr Syrsky, Ukraine’s top military commander, recently said his forces were fighting to hold back “one of the most powerful Russian offensives from launching a full-scale invasion.”

Ukraine got a boost on Sunday when the United States, after months of pressure from Kyiv, said it had granted permission for Ukraine to use American-provided weapons to fire deeper into Russia. On Tuesday, they used American-made ballistic missiles, called ATACMS (for Army Tactical Missile System), in an attack on a munitions depot in Russia.

But the election of Donald J. Trump to the American presidency this month injected an extra dose of uncertainty over the fate of the Ukrainian war effort.

While questions over whether the United States would continue to provide robust military support to Ukraine have resulted in a frenzy of diplomatic activity around the world, nowhere will those decisions be felt more acutely than on the front lines, where beleaguered Ukrainian troops are engaged in a fierce and bloody defense of their land.

Outnumbered by more than six to one along some stretches of the front, soldiers and commanders say they are hindered by a lack of combat infantry after years of heavy fighting and, just as important, by a shortage of experienced platoon and company commanders to lead untested recruits into battle. That has led to a fraying of Ukraine’s lines that has allowed Russia to make its largest gains since the first weeks of the war.

“Brigades that have been fighting for a long time are simply worn out,” Captain Viacheslav said, echoing concerns voiced by more than a dozen commanders and soldiers interviewed along the front last week.

The soldiers, identified only by their first names in accordance with military protocol, said they were speaking publicly about problems in the hopes of driving home the urgency of the moment to the military and civilian leadership as well as the public.

“We’re stretched thin,” Captain Viacheslav said. “People need to step up and serve. There’s no other way.”

As well as being short of personnel, Ukraine lacks the medium- and long-range weapons needed to conduct a consistent and effective campaign aimed at Russian logistics, command and control centers and other key targets.

More than a dozen Ukrainian soldiers on the front noted a marked decrease in artillery fire from their side in recent weeks, including the U.S.-made multiple rocket launching system known as HIMARS.

“HIMARS — I barely hear them at all anymore. They’re almost nonexistent,” said Sgt. Maj. Dmytro, a 33-year-old drone operator and company leader. “If we had more munitions, it could compensate for the lack of people.”

Given the shortage of artillery, drones now account for 80 percent or more of enemy losses along much of the front, commanders said.

That has made the drone operators prized targets. “It’s a constant struggle for survival — every day is a question of luck,” Sergeant Major Dmytro said.

A veteran drone pilot and platoon leader, Sgt, Maj. Vasyl, said the Russians were even dropping thousand-pound guided bombs to try to take out small drone teams, with one falling just a few hundred feet from his position last week.

“If they detect a drone operator, everything is thrown at us,” he said.

But drones alone, soldiers said, will not stabilize defensive lines.

“Nothing can replace infantry,” Captain Viacheslav said, adding that drones “cannot realistically stop the enemy.”

Russian forces are concentrating much of their efforts on capturing the last Ukrainian stronghold in the southern Donetsk region, Kurakhove, and opening a path to attack the strategic city of Pokrovsk from the south.

At the moment, Russia is still a long way from achieving the Kremlin’s aims of seizing Ukraine’s two most easternmost regions, Luhansk and Donetsk.

Despite their struggles, Ukrainian forces continue to make the Russians pay a high price for every advance, using their fleet of drones to slow the Russian onslaught.

“Our pilots and everyone working here knows that if we don’t stop them while they’re advancing, they’ll reach our positions 100 percent, and a gunfight will begin,” said Sergeant Major Vasyl. “It’s relentless: 24/7.”

He said he had taken part in some of the deadliest battles of the war but that the intensity of the Russian assaults in the southern Donbas was unlike anything he had witnessed.

“Once, they dropped off 30 infantry soldiers from an armored personnel carrier, and we took them all out in one spot,” Sergeant Major Vasyl said. “Another A.P.C. came in immediately after and unloaded 30 more soldiers. We lost count of how many times they sent more troops to the same spot. In half a day of fighting, the Russians lost more than 200 men.”

“In another six-hour clash,” he added, “we recorded a record 132 infantry killed.”

“These are staggering numbers,” Captain Viacheslav said.

But at the end of each engagement, the Russians took the land.

“If they’re willing to lose that many men just to advance, I’m not sure what could stop them,” he said.

His claims of Russian fatalities could not be independently verified.

Ukraine does not provide casualty figures, but soldiers say they also suffer grievous losses in each clash. Russian drone pilots attack them with the same ferocity with which the Ukrainians attack the Russians. The relentless attempts by Russia to storm Ukrainian trenches lead to deadly close-quarter combat that can favor the larger attacking side. And Russia has used its advantage in the air to pound Ukrainian fortifications with powerful guided bombs.

The Ukrainian soldiers shared drone video documenting the recent battles and allowed The New York Times to watch live video being streamed from the front at a command post a few miles away. Drone pilots targeted one group of Russian soldiers one after another, hour after hour.

While it was not possible to verify the precise death tolls, the scores of lifeless Russian soldiers scattered across fields, tree lines and roadsides offered a gruesome window into the extraordinary violence playing out across hundreds of miles of the front every day.

Ukrainian soldiers said the best way to stop the Russian advances was not by engaging in head-on clashes — which would always favor the larger Russian forces — but by weakening the enemy’s combat capabilities.

The lack of artillery compromises that effort. With no signs of the Russian offensive easing, Ukraine is racing to fortify defensive lines across the front. Tree lines are being cut down to limit places the Russians can hide. Tank traps are being dug deep into the ground. New trenches branch off from roadsides in all directions. And fertile fields are lined by concrete dragon’s teeth and seeded with mines.

But troops are still needed to fill the trenches.

Brigades normally charged with controlling a five-kilometer stretch of land are sometimes asked to hold a line two or three times as long, soldiers said.

When reinforcements are added, they lack combat experience, and each passing month, as Ukrainian losses mount, there are fewer battle-hardened veterans to help guide them.

Effective communication has also become an issue for Ukraine. When units from different brigades are dispatched to help fill gaps along the front, it can lead to a breakdown.

Junior Sgt. Denys, a drone operator working around Kurakhove, described an example of the problem.

When he detects enemy movement using a thermal imager, he only sees a heat signature.

“I don’t see the uniform and insignia,” he said.

To be sure he is not targeting friendly forces, he asks his commander if they have any troops in the area. But his commander needs to reach out to another battalion commander who in turn has to ask yet another.

“It takes time for this information to get back,” he said.

Time, however, is not a luxury soldiers under assault can afford.

Liubov Sholudko contributed reporting from eastern Ukraine.