The Israeli military campaign since October 7 has left more than 16,200 dead in Gaza.
At least 25 people were killed Tuesday in a strike that hit a southern Gaza school sheltering Palestinians displaced by war, the Hamas-run health ministry said.
Witnesses said dozens of injured people, as well as bodies recovered from under the rubble of Ma'an school in Khan Yunis had been taken to the city's Nasser hospital.
Khan Yunis, Gaza's second-largest city, has been targeted for several days by intense bombing by the Israeli army, which has now also sent ground troops to the area in its war on Hamas, sparked by the group's deadly October 7 attacks on Israel.
Mohammed Salou, whose sister was killed in Tuesday's strike, told AFP: "My cousin called me and told me to come because my sister's body was lying in a schoolyard, and we couldn't retrieve it."
He eventually managed to take the body to hospital, he said.
Salou said he believed "it was not the school itself which was targeted, but rather the area around it".
Many Palestinians have found refuge in schools or hospitals since the start of the Israeli offensive in October, believing they were relatively safe from attacks.
Israel has repeatedly claimed that Hamas uses civilians as "human shields" and that fighters from the Islamist movement which has ruled Gaza since 2007 are hiding among the displaced, including in schools and hospitals.
The UN humanitarian office OCHA says around 1.8 million people -- or 80 percent of the population of the narrow Palestinian territory -- have had to leave their homes during the war triggered by the unprecedented Hamas attacks, which according to Israeli authorities killed 1,200 people, mostly civilians.
The Israeli military campaign since October 7 has left more than 16,200 dead in Gaza, mostly women and children, according to the Hamas government.
Numerous schools and hospitals or their direct surroundings have been struck in the Gaza Strip since the war began.
"We took refuge in the school and they continued to target us," said Etraa al-Jerjawi, a woman who had left her home for fear of violence and lost a relative in the strike on Ma'an school.
On Sunday, the Israeli army published a map dividing the Gaza Strip into numbered "blocks", which it uses to warn civilians of imminent military operations.
OCHA has said the area designated on Sunday for "immediate evacuation" represented 20 percent of Khan Yunis and is home to around 117,000 people, half of whom already displaced.
As well as intense aerial bombardments, Israel launched a ground operation on October 27, initially in northern Gaza, which pushed a large part of the inhabitants towards the south.
But with the Israeli army now focusing on the south, international aid organisations warn civilians in the densely populated Strip are running out of places to flee to.
"Nowhere is safe in Gaza and there is nowhere left to go," said Lynn Hastings, UN humanitarian coordinator for the Palestinian territories.
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