At least 80 people have been killed in Israeli missile strikes on a school compound in Gaza City, according to the territory’s civil defence service, the latest in a string of attacks on schools that the Israeli army says are targeting militants using them as bases.

The bombing of Tabeen school, where about 6,000 displaced people were sheltering, was hit when many people were preparing for dawn prayers on Saturday, and reportedly caused a fire. Video from the scene showed horrific loss of life, with body parts, rubble and destroyed furniture scattered across blood-soaked mattresses.

Abu Anas, who helped to rescue the wounded, told the news agency Associated Press (AP): “There were people praying, there were people washing and there were people upstairs sleeping, including children, women and old people.

“The missile fell on them without warning. The first missile, and the second. We recovered them as body parts.”

Dr Fadel Naeem, the director of al-Ahli hospital in Gaza City, told the AP that the facility had received 70 bodies of those killed in the strikes and the body parts of at least 10 others.

In an interview with Al Jazeera, Naeem said Saturday was “one of the hardest days”.

“The situation at the hospital is catastrophic, with a severe shortage of medical supplies and resources due to the horrific Israeli massacre, which has resulted in numerous amputations and severe burns,” he said.

As it stands, the Tabeen school death toll is one of the largest from a single strike during 10 months of war between Israel and Hamas.

The Israel Defense Forces (IDF) said in a statement that the Palestinian claim was inflated, and at least 20 fighters, including senior commanders, were among the dead.

Israeli forces have targeted at least 10 schools since the beginning of July – including at one point four in four days – adding to the Gaza war’s staggering death toll, which is now approaching 40,000.

Israel blames Hamas for civilian casualties, saying that its fighters use civilian infrastructure as a cover, which makes buildings such as schools and hospitals valid targets. Hamas denies those claims.

Almost all of the strip’s 2.3 million population has been forced to flee their homes, often multiple times,during almost a year of fighting. Schools in particular have been used as shelters.

According to the civil defence service, three missiles targeted a two-storey building where women were using the top floor and men and boys the ground floor, which was also a space for prayer.

A Hamas political officer, Izzat el Reshiq, called the strikes a horrific crime and a serious escalation, adding in a statement that the dead did not include a “single combatant”.

The West Bank-based Palestinian Authority also made a rare statement on the attack. A spokesperson for the president, Mahmoud Abbas, urged the US – Israel’s most important diplomatic ally and weapons supplier – to “put an end to the blind support that leads to the killing of thousands of innocent civilians, including children, women, and the elderly”.

The EU foreign policy chief, Josep Borrell, said in a statement posted on X: “There’s no justification for these massacres.” The UK’s foreign minister, David Lammy, said he was “appalled”.

Jordan and Egypt also immediately condemned the attack, with Egypt’s foreign ministry saying that Israel’s “deliberate killing” of Palestinians proves a lack of political will to end the war in Gaza.

Egypt, along with the US and Qatar, called this week for Israel and Hamas to resume negotiations to finalise a ceasefire and hostage-release deal, saying there were no excuses “from any party for further delay”.

The Israeli prime minister, Benjamin Netanyahu, said Israel would send a delegation to the talks beginning on 15 August. His administration has been accused of repeatedly sabotaging ceasefire talks.

There has been no response yet from Hamas, and is unclear if the latest deadly strike will affect the militant group’s position.

Iran joined the chorus of condemnation later on Saturday, calling the Tabeen attack “barbarous” and “a war crime”.

Both Iran and the powerful Lebanese militant group Hezbollah have vowed revenge against Israel for the back-to-back assassinations of Hezbollah commander Fuad Shakur and Hamas’s political chief, Ismail Haniyeh, on 31 July. Israel has not commented on Haniyeh’s death, but its spy agency has a history of targeted killing operations abroad.

Israel is still bracing for retaliatory attacks, amid fears the Gaza war is on the brink of morphing into a region-wide conflict.

Hamas triggered the fifth war with Israel since it seized control of the Gaza Strip in 2007 with its 7 October attack on communities across southern Israel, in which about 1,200 people were killed and another 250 taken hostage.

A ceasefire brokered at the end of November saw about 100 Israelis released in exchange for about 200 women and children held in Israeli jails, but broke down after a week.