The Israeli military said the strike on Thursday hit a school complex in Jabaliya that was serving as a command and control center for Hamas and Islamic Jihad. The military said it had killed dozens of fighters in the strike. There were reports that more than 100 people were injured.

Rescue workers, Palestinian media and at least one civilian who had been living there said the site had housed civilians. Juliette Touma, a spokeswoman for UNRWA, the U.N. agency that assists Palestinian refugees, confirmed that one of its schools in Jabaliya was struck on Thursday.

“We received reports that among the scores of people killed are children, which is often the case,” Ms. Touma said, adding that the school was the third UNRWA site to be attacked in the last week. She said the U.N. had not been able to determine the accuracy of Israel’s claims that Hamas and other groups were active in its school compounds.

“We call for an independent investigation and commissions of inquiry to look into these reoccurring claims,” she said. “We call once again on all Palestinian armed groups, including Hamas, to refrain from using UNRWA facilities for military or fighting purposes.”

The Gaza government media office said the strike had killed 28 people and injured 160 more. One man who had been staying in the school compound, Mahmoud Hamdouna, 29, said he left the building just minutes before the strike to search for food. He ran back when he heard the explosion and found a scene that was “horrific, a real massacre,” he said.

“I saw around 25 dead bodies and so many injured,” said Mr. Hamdouna, who had fled to Jabaliya from Beit Hanoun, another town in Gaza’s besieged north. “People lost limbs at the scene and many had serious head injuries. Many others are still under the rubble.”

Satellite imagery captured three days before the strike showed many tents inside the school’s courtyard.

Regular schooling has not happened in the Gaza Strip since the start of the war last October. Instead, displaced people have sought shelter at disused school buildings across the enclave, crowding into classrooms, corridors and playgrounds.

But school buildings have also been a frequent target for airstrikes because Israel argues that many have been used by Hamas to house fighters, store equipment or plan attacks. A vast majority of school buildings, including every university in the Gaza Strip, has been damaged or destroyed during the war.

The Israeli military said in a statement that before the airstrike on Thursday it had taken “numerous steps” to “mitigate the risk of harming civilians, including the use of precise munitions, aerial surveillance and additional intelligence.”

“This is a further example of the Hamas terrorist organization’s systematic abuse of civilian infrastructure in violation of international law,” the military said.

Lauren Leatherby contributed reporting.

The militant was killed on Wednesday during a firefight in southern Gaza with Israeli soldiers, according to three of the officials. The remains of the body will be compared to DNA collected from Mr. Sinwar during his decades-long incarceration inside Israeli jails, two officials said.

U.S. officials said that Israel had told the Americans that its soldiers may have killed Mr. Sinwar. All the officials spoke on the condition of anonymity in order to discuss a sensitive matter. Hamas made no immediate comment.

The death of Mr. Sinwar, an architect of the Hamas-led attack last October that set off the war in Gaza, would raise hopes of an end to the conflict. Both Mr. Sinwar and the Israeli government had refused to compromise during the monthslong negotiations for a truce. His death could either prompt Hamas to agree to some of Israel’s demands — or provide Benjamin Netanyahu, the Israeli prime minister, with a symbolic victory that would give him the political cover he needs to soften his own negotiating stance.

The military statement on Thursday said that three militants were killed in the exchange and that there were no signs of hostages nearby. It added that soldiers continued to operate in the area.

Israeli officials said they believed the body may be Mr. Sinwar’s because of the physical similarities, and because intelligence had suggested he was hiding in southern Gaza.

But they also urged caution because no DNA test had yet been carried out and because they were surprised that such a senior Hamas leader would have been caught in a gunfight above ground. Mr. Sinwar, who is in his early 60s, was thought to have been killed on previous occasions, only for the initial conjecture to be proven wrong.

Israel and the United States both invested huge resources in the hunt for Mr. Sinwar. American spy agencies formed a targeting cell after last October’s attacks in Israel to study him and try to intercept his communications.

In late January, Israeli and American officials thought they were on the verge of catching him when Israeli commandos raided an elaborate tunnel complex in southern Gaza where they thought he was hiding, according to American and Israeli officials. But Mr. Sinwar had moved from the bunker beneath the city of Khan Younis just days earlier, leaving behind documents and stacks of Israeli shekels totaling about $1 million, the officials said.

In August, Israeli troops who discovered the bodies of six hostages in a warren of tunnels beneath Rafah, in southern Gaza, found signs of Mr. Sinwar’s past presence in the area, according to Yoav Gallant, the Israeli defense minister.

Mr. Sinwar’s decision to attack Israel on Oct. 7 has proved one of the most fateful moves in recent Middle East history. Roughly 1,200 people were killed in Israel some 250 others abducted in the assault, which set off a devastating Israeli counterattack in Gaza in which more than 42,000 Palestinians have been killed and much of the blockaded territory left in ruins.

It also led to a broader war between Israel and Hamas’s regional allies, including Iran, the Houthis in Yemen and Hezbollah in Lebanon, where Israel is currently mounting extensive ground and air attacks.

Mr. Sinwar has been in hiding throughout the war, eschewing electronic devices and relying on a network of couriers to stay in touch with his organization, according to Israeli and U.S. officials. At times, his communications with Hamas have been intermittent, prompting repeated speculation about his fate.

But he remained by far Hamas’s most important figure, with the group’s representatives at cease-fire negotiations in Qatar this year telling diplomats that they needed Mr. Sinwar’s input before taking major decisions. In recent months U.S. officials came to believe Mr. Sinwar did not want to reach a deal and was determined to see Israel enmeshed in a broader regional war.

Mr. Sinwar emerged from two decades of prison in Israel to rise to the helm of Hamas in Gaza in 2017. In August, he was also named political leader of the entire movement, both inside and outside Gaza, after Israel assassinated his predecessor Ismail Haniyeh, consolidating Mr. Sinwar’s influence over the organization.

Julian E. Barnes and David E. Sanger contributed reporting.

“This was a unique demonstration of the United States’ ability to target facilities that our adversaries seek to keep out of reach, no matter how deeply buried underground, hardened or fortified,” Mr. Austin said in a statement late Wednesday night. “The employment of U.S. Air Force B-2 Spirit long-range stealth bombers demonstrate U.S. global strike capabilities to take action against these targets when necessary, anytime, anywhere.”

A statement from U.S. Central Command on Wednesday night said that U.S. Navy “assets” also took part in the attack, which the unit, based in Tampa, Fla., said was launched against “various advanced conventional weapons used to target U.S. and international military and civilian vessels navigating international waters.”

Attacking so-called hardened buried sites generally requires the use of specially built bombs that have much thicker steel cases and contain a smaller amount of explosives than similarly sized general-purpose bombs. The heavy casings of such “bunker buster” bombs allows the munition to stay intact as it punches through soil, rock or concrete before detonating.

The B-2 is the only warplane that can carry the largest of this class of weapon in the Pentagon’s inventory: A 30,000-pound GPS-guided munition called the GBU-57 Massive Ordnance Penetrator, or MOP, that contains the equivalent of about 5,600 pounds of TNT. A Pentagon spokesman declined to say whether that weapon was used in the attack on Wednesday.

The Air Force had only acknowledged building 20 such bombs as of 2015, according to publicly available documents, and five were expended in testing at White Sands Missile Range in New Mexico in 2012. According to an Air Force site, the weapon is capable of reaching targets up to 200 feet underground before exploding.

The U.S. arsenal also includes 5,000-pound and 2,000-pound penetrator bombs that can be dropped by other warplanes.

The Air Force is believed to have just 19 operational B-2 bombers, all of which are permanently based at Whiteman Air Force Base in Missouri, though the Pentagon has occasionally deployed some for exercises in the Pacific and Indian Ocean.

For B-2 bombers to take part in Wednesday’s attack, the aircraft would have either had to fly round-trip from Missouri to Yemen and refuel midair, or take off from a base much closer to their targets.

“Due to operational security, we won’t discuss our operating locations within the region,” Maj. Gen. Patrick S. Ryder, the Pentagon spokesman, said in response to a question about where the B-2s were launched for Wednesday’s attack.

The Houthis started targeting civilian tankers and cargo ships at sea in solidarity with Hamas last year. U.S. forces have shot down dozens of Houthi attack drones and anti-ship missiles launched at commercial merchant vessels since mid-November and have also frequently launched airstrikes against Houthi missile and radar sites.

Many also denounced Israel’s strike of a hospital complex on Monday that ignited fires that spread to a tent shelter for displaced people next to a hospital, leading to accounts that people were burned alive.

Some of the sharpest criticism came from Israel’s allies, including the United States, which vetoed three resolutions blocking efforts for a cease-fire in the first eight months of the war and abstained from one that eventually passed in June.

“There are no words, simply no words, to describe what we saw,” said Linda Thomas-Greenfield, the United States ambassador to the United Nations. “Israel has a responsibility to do everything possible to avoid civilian casualties, even if Hamas was operating near the hospital in an attempt to use civilians as human shields. We have made this clear to Israel.”

Ms. Thomas-Greenfield also said that a “policy of starvation” in northern Gaza would be “horrific and unacceptable and would have implications under international law and U.S. law.”

The rebuke came as the United States warned Israel on Sunday in a letter that if it did not provide scaled-up humanitarian aid to Gaza in the next 30 days, it would face repercussions that could include a halt to American military aid to Israel.

Riyad Mansour, the Palestinian envoy to the United Nations, accused Israel of trying to force Palestinians out of Gaza by withholding food aid, saying, “This is not war; these are crimes.” Mr. Mansour added: “This is genocide. They must be stopped, and they must be stopped now.”

Israel’s ambassador to the United Nations, Danny Danon, defended Israel’s actions and blamed Hamas for using Palestinian civilians as human shields and for robbing aid trucks.

“The only way to improve the humanitarian situation in Gaza is to defeat Hamas and completely disarm it,” Mr. Danon said. He claimed Hamas “steals, stores and sells the aid that enters the Gaza Strip and uses it to feed its terrorist machine and not to feed the Gazans.”

Scott Peterson, the Gaza director of UNRWA, the main U.N. aid agency for Palestinians, said on Wednesday that Israel had allowed 12 aid trucks into northern Gaza on Sept. 30, but no other trucks were allowed into that region of the enclave after that.

COGAT, the Israeli government agency that oversees policy in Gaza and the West Bank, insisted that it was not limiting aid to Gaza and has accused humanitarian agencies of failing to distribute the supplies it has admitted into the enclave after screening. On Wednesday, it said that it had inspected and permitted 50 aid trucks to enter northern Gaza from Jordan — carrying food, water, medical and other supplies — “in accordance with international law.”

To service civilians adequately, the United Nations said it needed about 40 to 50 trucks per day in northern Gaza and about 250 to 300 aid trucks per day in the south.

“The level of suffering in Gaza defies our ability to capture it in words, or even to comprehend its scale,” said Joyce Msuya, the U.N.’s acting humanitarian chief to the Council. “Reality is brutal in Gaza, and it gets worse every day, as the bombs continue to fall, as fierce fighting continues unabated and as supplies essential for people’s survival and humanitarian assistance are blocked at every turn.”

Ms. Msuya told the Council that Israel’s military on Monday had detained U.N. aid workers who were evacuating severely burned patients to other hospitals in Gaza, holding them up at checkpoints for hours and threatening the lives of the injured.

“Medical staff kept one child alive by hand-pumping oxygen for over seven hours until they made it through the checkpoint,” she said.

Mr. Danon did not respond to her comments when addressing the Council.