DEIR AL-BALAH, Gaza Strip — An Israeli airstrike hit a school-turned-shelter in Gaza early Saturday, killing at least 80 people and wounding nearly 50, Palestinian health authorities said, in one of the deadliest attacks of the 10-month war between Israel and Hamas.

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 targeting 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 that 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.”

Video from the scene showed walls blown out on the ground level of a large building. Concrete chunks and twisted metal lay on the blood-soaked floor, along with clothing, furniture and other debris. 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, said the facility received 70 bodies along with the body parts of at least 10 more. Gaza’s Health Ministry said an additional 47 people were wounded.

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

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

“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 his 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 housing the mosque, and the school on the second — 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.

“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 he was “appalled.”

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

Israel has blamed civilian deaths in Gaza on Hamas, saying the group endangers noncombatants by using schools and residential neighborhoods as bases for operations. The U.N. human rights office acknowledged that co-locating combatants with civilians is a violation of international humanitarian law but said 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 the strike on the school showed that Israel has no intention of reaching a cease-fire deal.