DEIR AL-BALAH, Gaza Strip — An Israeli strike early Thursday on a school sheltering displaced Palestinians in central Gaza killed more than 30 people, including 23 women and children, according to local health officials. The Israeli military said that Hamas militants were operating from within the school.

It was the latest instance of mass casualties among Palestinians trying to find refuge as Israel expands its offensive. A day earlier, the military announced a new ground and air assault in central Gaza, pursuing Hamas militants it says have regrouped there. Troops repeatedly have swept back into parts of the Gaza Strip they have previously invaded, underscoring the resilience of the militant group despite Israel's nearly eight-month onslaught .

Witnesses and hospital officials said the predawn strike hit the al-Sardi School, run by the United Nations agency for Palestinian refugees known by the acronym UNRWA. The school was filled with Palestinians who had fled Israeli operations and bombardment in northern Gaza, they said.

Ayman Rashed, a man displaced from Gaza City who was sheltering at the school, said the missiles hit classrooms on the second and third floor where families were sheltering. He said he helped carry out five dead, including an old man and two children, one with his head shattered open. “It was dark, with no electricity, and we struggled to get out the victims,” Rashed said.

Casualties from the school strike arrived at the Al-Aqsa Martyrs Hospital in nearby Deir al-Balah, which had already been overwhelmed by a stream of constant ambulances since the central Gaza incursion began 24 hours earlier, said Omar al-Derawi, a photographer working for the hospital.

Videos circulating online appeared to show several wounded people being treated on the floor of the hospital, a common scene in Gaza’s overwhelmed medical wards. Electricity in much of the hospital is out because staff are rationing fuel supplies for the generator.

“You can't walk in the hospital — there’s so many people. Women from the victims’ families are massed in the hallways, crying,” he said.

Hospital records and an Associated Press reporter at the hospital recorded at least 33 dead from the strike, including 14 children and nine women. Another strike on a house overnight killed six people, according to the records. Both strikes occurred in Nuseirat, one of several built-up refugee camps in Gaza dating to the 1948 war surrounding Israel’s creation , when hundreds of thousands of Palestinians fled or were driven from their homes in what became the new state.

Footage showed bodies wrapped in blankets or plastic bags being laid out in lines in the courtyard of the hospital. Mohammed al-Kareem, a displaced Palestinian sheltering near the hospital, said he saw people searching for their loved ones among bodies, and that one woman kept asking medical workers to open the wraps on the bodies to see if her son was inside.

"The situation is tragic," he said.

Philippe Lazzarini, the commissioner-general of UNRWA, said in a post on X that 6,000 people were sheltering in the school when it was hit without prior warning. He said UNRWA was unable to verify claims that armed groups were inside.

An Israeli military spokesman, Lt. Col. Peter Lerner, said that the army wasn't aware of any civilian casualties in the strike. He said that intelligence indicated that militants had used the school compound to orchestrate some of the attacks on Oct. 7 and that at least 20 militants there were using it currently as a “staging realm” to launch attacks on Israeli soldiers. The military gave no evidence for its claims and released a photo of the school, pointing to classrooms on the second and third floor where it claimed militants were located.

The military said it took steps before the strike "to reduce the risk of harming uninvolved civilians ... including conducting aerial surveillance, and additional intelligence information.”

UNRWA schools across Gaza have functioned as shelters since the start of the war, which has driven most of the territory's population of 2.3 million Palestinians from their homes.

Last week, Israeli strikes hit near an UNRWA facility in the southern city of Rafah, saying they were targeting Hamas militants. An inferno ripped through tents nearby housing displaced families , killing at least 45 people . The deaths triggered international outrage, and Israeli Prime Minister Benjamin Netanyahu said the fire was the result of a “tragic mishap.” The military said the fire may have been caused by secondary explosions. The cause of the explosions has not been determined.

Israel sent troops into Rafah in early May in what it said was a limited incursion, but those forces are now operating in central parts of the city. More than 1 million people have fled Rafah since the start of the operation, scattering across southern and central Gaza into new tent camps or crowding into schools and homes.

Israel launched its campaign in Gaza after Hamas’ Oct. 7 attack into Israel, in which militants killed some 1,200 people and took another 250 hostage. Israel's offensive has killed at least 36,000 Palestinians, according to Gaza's Health Ministry , which does not distinguish between fighters and civilians in its figures.

Israel blames civilian deaths on Hamas because it positions fighters, tunnels and rocket launchers in residential areas.

The United States has thrown its weight behind a phased cease-fire and hostage release outlined by President Joe Biden last week. But Israel says it won’t end the war without destroying Hamas, while the militant group is demanding a lasting cease-fire and the full withdrawal of Israeli forces.

Far-right members of Netanyahu's government have threatened to bring down the coalition if he signs onto a cease-fire deal.

Israel has routinely launched airstrikes in all parts of Gaza since the start of the war and has carried out massive ground operations in the territory’s two largest cities, Gaza City and Khan Younis, that left much of them in ruins.

The military waged an offensive earlier this year for several weeks in Bureij and several other nearby refugee camps in central Gaza.

Troops pulled out of the Jabaliya camp in northern Gaza last Friday after weeks of fighting caused widespread destruction. First responders have recovered the bodies of 360 people, mostly women and children, killed during the battles.