The Al-Jawni school in Nuseirat had already been bombed several times over the course of the 11-month war in Gaza.

The latest strike on Wednesday flattened part of the UN-run facility where Gazans had sought shelter, leaving only a charred heap of rebar and concrete.

"For the fifth time, Israeli forces bombed the UNRWA-run Al-Jawni School, killing 18 citizens, including two UNRWA staff members, children, and women, and injuring more than 18 others," Gaza's Civil Defence spokesperson Mahmud Bassal posted on Telegram, referring to the UN agency for Palestinian refugees.

UNRWA later said six of its staffers had been killed in two Israeli air raids on the Nuseirat school and its surroundings, calling it the highest death toll among its team in a single incident.

"Among those killed was the manager of the UNRWA shelter and other team members providing assistance to displaced people," the UN agency posted on X.

"Schools and other civilian infrastructure must be protected at all times, they are not a target."

Across the besieged strip, many school buildings have been repurposed to shelter displaced families as the vast majority of Gaza's 2.4 million people have been repeatedly uprooted by the war.

Israeli forces have struck several such schools in recent months.

Survivors of the strike scrambled to retrieve bodies and belongings from the rubble, telling AFP they had to step over "shredded limbs".

"I can hardly stand up," said one man, holding a plastic bag of human remains.

"We've been going through hell for 340 days now, what we've seen over these days, we haven't even seen it in Hollywood movies, now we're seeing it in Gaza."

 'Senseless killing'

Gaza has repeatedly been called the world's deadliest place for humanitarian workers, with aid organisations repeatedly faulting the "deconfliction" process -- the coordination of movements with military parties.

In a statement, the UN said that the Nuseirat school had been "deconflicted" earlier on Wednesday.

UNRWA head Philippe Lazzarini said after the school strike that at least 220 members of the agency's staff have been killed in Gaza.

"Endless & senseless killing, day after day," he posted on X.

"Humanitarian staff, premises & operations have been blatantly & unabatedly disregarded since the beginning of the war."

UN chief Antonio Guterres called what is happening in Gaza "totally unacceptable".

Months of behind-the-scenes negotiations mediated by Qatar, Egypt and the United States have so far failed to secure a truce.

A Hamas delegation met Qatari and Egyptian mediators in Doha on Wednesday, the group said in a statement, which did not indicate whether there was a breakthrough.

Recent rounds of mediation held in Doha and Cairo have tried to hash out a framework laid out in May by US President Joe Biden, but both Israel and Hamas have publicly signalled deeper entrenchment in their negotiating positions.