An Israeli airstrike hit a school-turned-shelter in Gaza early Saturday, killing at least 80 people and wounding nearly 50 others, Palestinian health authorities said, in one of the deadliest attacks of the 10-month Israel-Hamas war. A witness said it struck during prayers at a mosque in the building.

It was the latest of what the U.N. human rights office called “systematic attacks on schools” by Israel, with at least 21 since July 4 leaving hundreds dead, including women and children.

“For many, schools are the last resort to find some shelter," it said after Saturday's attack.

The Israeli military acknowledged it targeted the Tabeen school in central Gaza City, saying it hit a Hamas command center in a mosque in its compound and killed 19 Hamas and Islamic Jihad fighters. Izzat al-Rishq, a top Hamas official, denied there were militants in the school.

Israel's military also disputed the toll, saying the “precise munitions” used “cannot cause the amount of damage that is being reported” by the Hamas-run government. It said the steps it took to limit the risk to civilians included the use of a “small warhead,” aerial surveillance and intelligence information.

Walls were blown out on the ground level of the large building. Concrete chunks and twisted metal lay on the blood-soaked floor. Bodies, some in bloodstained shrouds, were placed shoulder to shoulder in makeshift graves, making room for more.

Fadel Naeem, director of the al-Ahli hospital in Gaza City, told The Associated Press that it received 70 bodies along with the body parts of at least 10 others. Gaza's Health Ministry said that another 47 people were wounded.

“We received some of the most serious injuries we encountered during the war,” he said, with many wounded having limbs amputated and some with severe burns.

The strike hit without warning before sunrise as people prayed, according to witness Abu Anas.

“There were people praying, there were people washing and there were people upstairs sleeping, including children, women and old people," he said, prayer beads in hand. “The missile fell on them without warning. The first missile, and the second. We recovered them as body parts.”

Three missiles ripped through the two-story building — the first floor housed the mosque, and the second level had a school — where about 6,000 displaced people were taking shelter, said Mahmoud Bassal, a spokesperson for the Civil Defense first responders, who operate under the Hamas-run government.

Many of the casualties were women and children, he said.

A camera operator working for the AP said a missile appeared to have penetrated the floor of the classrooms to the mosque below and exploded.

The U.N. previously said that as of July 6, 477 out of 564 schools in Gaza had been directly hit or damaged in the war, adding that Israel has a duty under international law to provide safe shelter for the displaced.

“There’s no justification for these massacres," European Union foreign policy chief Josep Borrell said in a statement posted on X, referring to strikes on schools. U.K. Foreign Minister David Lammy said that he was “appalled." France's foreign ministry called the recent number of civilian victims in Israeli strikes on schools “intolerable.”

The U.S. said it was deeply concerned about reports of civilians killed.

“Far too many civilians continue to be killed and wounded,” U.S. National Security Council spokesperson Sean Savett said in a statement.

Israel has blamed civilian deaths in Gaza on Hamas, saying the group endangers people by using schools and residential neighborhoods as bases for operations. The U.N. human rights office acknowledged that colocating combatants with civilians is a violation of international humanitarian law, but that Israel must also comply with the law’s principles of precaution and proportionality.

The strike came as U.S., Qatari and Egyptian mediators renewed their push for Israel and Hamas to achieve a cease-fire agreement that could help calm soaring tensions in the region following the assassination of Hamas political leader Ismail Haniyeh in Tehran and a senior Hezbollah commander in Beirut.

Egypt, which borders Gaza, said that the strike on the school showed that Israel had no intention of reaching a cease-fire deal. Neighboring Jordan condemned the attack as a “blatant violation” of international law. Qatar demanded an international investigation, calling it a “heinous crime” against civilians.

Vice President Kamala Harris, speaking to reporters traveling with her in Phoenix, Arizona, on Saturday, said of the Israeli strike in Gaza: “Yet again, far too many civilians have been killed.”

“Israel has a right to go after the terrorists that are Hamas,” she said. “But as I have said many, many times they also have, I believe, an important responsibility to avoid civilian casualties.”

Pressed on the fact that such comments have done little to lower the numbers of civilians in Gaza killed in recent months, Harris said, “First and foremost — and the president and I have been working on this around the clock — we need to get the hostages out.”

“We need a hostage deal and we need a cease-fire,” she said. “And I can’t stress that strongly enough. It needs get to done. The deal needs to get done and it needs to get done now.”

Late on Friday, two separate airstrikes in central Gaza killed at least 13 people, including three children and seven women, hospital authorities said. An AP journalist counted the bodies at the al-Aqsa Martyrs hospital in the central city of Deir al-Balah.

One strike hit a house in the Nuseirat refugee camp, killing seven people, all but one of them women, hospital officials said. Another hit a house in Deir al-Balah, killing six, including a woman and her three children, the hospital said.

Israel’s campaign in Gaza has killed more than 39,790 Palestinians and wounded more than 92,000 others, according to the Health Ministry, which doesn't distinguish between combatants and civilians in its tally. The war was triggered by Hamas’ Oct. 7 attack in which militants from Gaza stormed into southern Israel, killing around 1,200 people and abducting 250 others.

Families of hostages demonstrated again Saturday night in Tel Aviv seeking a cease-fire deal to bring loved ones home.

More than 1.9 million of Gaza’s prewar population of 2.3 million have been driven from their homes, fleeing repeatedly across the territory to escape offensives. Most are crowded into tent camps in an area of about 50 square kilometers (19 square miles) on the Gaza coast with few basic services or supplies.

In the occupied West Bank, dozens of people gathered in Ramallah to protest the latest Israeli strike on a school.

“The message that must be sent to the world, a numb world, a world that is not moving, is ‘how long will the war continue?’” asked one, Muin Barghouti.