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 Gaza, as fighting there enters a 14th month and a humanitarian crisis intensifies.

Fourteen Security Council members voted for the resolution, and only the United States voted against it.

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 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 is implemented.

The veto was the fourth time the United States blocked an effort by the Council to demand a cease-fire since the war began over a year ago when Hamas attacked Israel and took more than 200 people hostage. In the months since, more than 40,000 people have been killed in Gaza, according to the local health authorities.

The veto comes as Washington has been working for months to help negotiate a cease-fire between the parties and a deal to release the hostages.

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

The 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 Palestinian aid agency UNRWA to carry out its work in the territory.

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

“It is a sad day for the Security Council, for the United Nations and for the international community,” said Algeria’s ambassador, Amar Bendjama. He said the 14 members who supported the resolution had spoken for the wider international community.

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

The Security Council, whose permanent members are divided over the wars in Ukraine and Gaza, has struggled to speak in one voice and play an effective role in mediating or ending these conflicts. The deadlock over Gaza in the Council has been the result of U.S. policy and has invited criticism of staunch U.S. support for Israel and frustrated the wider U.N. membership, including some of America’s closest allies, the permanent council members, Britain and France.

The Council has tried to bring the war in Gaza to the table for action in the past year with multiple resolutions. The United States blocked three previous resolutions calling for a cease-fire and release of hostages saying at the time that Israel had the right to defend itself and it was not yet time for the war to end. Russia and China vetoed an American resolution in March that called for “an immediate and sustained cease-fire,” in a vote in which Algeria joined them and Guyana abstained.

In March the United States abstained from voting on a resolution that called for a temporary halt to the fighting for the month of Ramadan.

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 move is also noteworthy because it is part of a series of late 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.

A top U.S. envoy met with Lebanese officials for a second day on Wednesday, continuing an unusually long visit amid cautious optimism over a potential cease-fire agreement in the war between Israel and the Lebanese militant group Hezbollah.

Speaking at a news conference in Beirut, Amos Hochstein, the senior Biden administration envoy, said that there had been “additional progress” as a result of the latest discussions, and that he would travel to Israel later on Wednesday “to try to bring this to a close if we can.”

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

Mr. Hochstein spoke to reporters in Beirut after meeting 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 the extended visit raised hopes that negotiations could be inching forward.

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 efforts in Lebanon, and that he did not believe the transfer of power in Washington would imperil efforts to bring about peace.

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

On Tuesday, Mr. Hochstein said that the gaps between Israel and Hezbollah had “narrowed” in discussions in recent weeks, though ultimately any results from the negotiations would be “the decision of the parties.” He declined to discuss details of the negotiations.

Israel’s war 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.

Here are other developments:

• 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 and Matthew Mpoke Bigg contributed reporting.

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.

It used to be like almost any other national carrier, fielding gripes about flight delays, ticket prices and bad food.

But since Israel’s recent invasion of Lebanon to battle Hezbollah, Middle East Airlines has been elevated to an unexpected national hero — its planes taking off and landing only hundreds of yards away from the bombings rocking Beirut.

Israeli bombs have taken out the top leaders of Hezbollah, Lebanon’s powerful Iran-backed militant group. Israel’s military has flattened entire Lebanese districts. And through it all, the airline, known to locals by the abbreviation MEA, has continued flying, sometimes just minutes after bombs have pummeled the road leading to Beirut’s international airport. It is the only commercial airline still operating in and out of Lebanon.

In a country hollowed out by corrupt leaders, with no army strong enough to defend it, MEA has become a source of pride for a population with few champions left to cheer.

Local news channels have broadcast music videos paying tribute to the airline. Officials heap it with praise. And even as some passengers complain of soaring prices, countless accounts on Lebanese social media have waxed lyrical.

“Heroes of the sky,” one person posted.

“We are steadfast and gallant, like the gallantry of the Lebanese Middle East Airlines that flies above Israeli bombing,” a prominent Muslim cleric wrote on X.

It is all a bit much for Mohammed Aziz, a spokesman for the airline, who is eager to manage expectations: “We are not the military. We are not heroes. We are a commercial airline,” Mr. Aziz, a former MEA captain, told The New York Times.

Yet MEA could be considered a metaphor for resilience throughout the history of a nation that has known too many wars.

It managed to fly through the 1967 and 1973 regional wars, as well as the 15-year civil war that ended in 1990. It kept flying during the 1982 Israeli invasion, and Israel’s 2006 war with Hezbollah.

MEA has become a rare success story in Lebanon — it has remained largely profitable — in spite of the country’s repeated collapse into crises.

“We always have Plan A, B, C, D and even E,” Mr. Aziz said.

Since Israel launched its invasion, the key to keeping planes safe has been a “crisis unit” established by MEA and the country’s Civil Aviation Authority, said Mazen Sammak, the head of Lebanon’s private pilot association and an airline safety consultant. They assess every flight, he said, and if a risk is perceived, they either delay takeoffs or tell pilots nearing Lebanon to slow their approach until it is safer to land.

“Up to now, we’ve had what has become a famous phrase: assurances, but no guarantees,” he said. “These are not normal circumstances to fly or operate an airport — you can’t find another airport worldwide operating with shelling hitting 500 meters away from it.”

The airline’s persistence has helped give this tiny Mediterranean nation — surrounded by Syria, Israel and the sea — a sense that it is not wholly cut off from the world.

“As long as the airport is open, it means we’re not totally screwed,” said Makram Rabah, a historian at the American University of Beirut. “There’s a Mediterranean element here. People living in the Mediterranean always like to venture out: Whether it was our seafaring ancestors, or our modern form of transport, by air. You cannot disconnect Lebanon from the world.”

It is not just the Lebanese who see the flagship carrier as that symbolic link — so did Israel, said Mr. Rabah, who writes about modern Lebanese history.

He pointed to Israel’s 1968 raid on the Beirut airport in retaliation for the hijacking of an Israeli plane, led by Palestinian militants based in Lebanon. Israel blew up more than half of the MEA fleet, though the airline had no known links to the hijacking.

MEA was actually able to recover its losses because of an enterprising decision to buy risk insurance — not for Middle Eastern conflicts, but to continue flights to India and Pakistan during a 1965 war between those nations.

The airline, like the nation it represents, was a source of international fascination as early as the 1960s — a period seen as Lebanon’s glamorous heyday, when it was a playground for celebrities and playboys who arrived on yachts and private jets.

Time magazine marveled at the company’s success in a region of heavily subsidized, government-owned national carriers, calling it “the most successful Arab aerial enterprise since the flying carpet.”

The publication gave the airline’s chief executive at the time, Najib Alamuddin, the nickname the “flying sheikh.” Mr. Alamuddin, a Druse man with the religious title sheikh, even used it as the title of his autobiography.

Within Lebanon, Mr. Alamuddin, who died in 1996, became the first of many MEA executives thrust into the unlikely role of national negotiator to protect the airline, Mr. Aziz said. Mr. Alamuddin met with the country’s constantly shifting constellation of militant factions, Mr. Aziz said, to ensure they did not attack the airline.

“Back then, it was a little more risky. We had to coordinate with 30 different factions,” Mr. Aziz said. “It was really much more difficult than now, with only two parties,” he said — Israel and Hezbollah.

Today’s chairman, Mohammed al-Hout, has mostly negotiated with the United States Embassy, which communicates with Israel, and with Lebanese officials close to Hezbollah, Mr. Aziz said. That is to ensure that the militant group refrains from trying to use the airport, and that Israel does not attack it. Mr. al-Hout declined to be interviewed.

Mr. Aziz insisted that despite the dramatic images today, MEA would not risk the safety of its passengers or crew, and that its decision to fly was the result of decades-long experience navigating conflict.

“Half of my life was spent in emergency planning. We have quite a lot of experience in this,” he said. “It’s what makes people trust our risk assessment.”

From the civil war until today, MEA has operated through conflict with a careful and ever-changing orchestration of flights that ensures only a certain number of planes are on the ground in Beirut at any given time, to distribute insurance risks.

After Israel attacked and invaded Lebanon in 1982, Mr. Aziz was among the pilots who flew the airline’s jets to Cyprus, taking off using only half of the runway to avoid sections cratered by bombs. It was his son’s turn to do the same as a freshly minted pilot during Israel’s 2006 war against Hezbollah.

Today, both his son and the son of the chief executive, Mr. al-Hout, are among the pilots who continue flights in and out of Beirut, Mr. Aziz said.

Despite MEA’s safety measures, Mr. Sammak, the airline consultant, said that continuing to operate put “big psychological pressure” on the airline’s pilots, who are trained every six months on emergency takeoffs and landings, but not for an emergency caused by armed conflict. “What if that happened during shelling, or during bombardment?” he said.

Perhaps the only ones who have a tougher job, he said, are those who work in the airport’s glass control towers “with shelling all around them.”

It is these dangers that have made some Lebanese cringe at the fervor over the airline’s continued feat of flight, exasperated that Lebanon’s reputation for resilience obscures generations of trauma.

“Of course they are resilient — not because they want to be resilient, but because they have no choice but to be resilient,” said Fawaz Gerges, a Lebanese American historian and professor at the London School of Economics.

As a child of Lebanon’s 1958 war, Mr. Gerges said he had grown up hearing Lebanese people asking “what war generation are you?”

“Resilience comes with a very high mental and emotional cost,” he said. “Most of us Lebanese are deeply scarred.”

Hwaida Saad contributed reporting.

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.

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 embassy rarely issues such a specific alert or shuts down.

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.