At least 12 people were killed and 75 injured when a UN facility sheltering civilians was struck in Khan Younis in southern Gaza, the UN’s Palestinian refugee agency says.
UNRWA said two shells hit its Khan Younis Training Centre during fighting in the city’s western outskirts.
Its commissioner condemned the “blatant disregard of basic rules of war”.
Israel’s military said it had ruled out that the incident was the result of an air or artillery strike by its forces.
It added that it was reviewing Israeli operations nearby and examining the possibility that it was “Hamas fire”.
Israeli troops have been battling Hamas fighters as they advance into western Khan Younis, a day after the military said it had completely encircled the city.
Clashes and bombardment around the city’s two main hospitals have also left thousands of patients, staff and others unable to leave.
The conflict was triggered by an unprecedented cross-border attack by Hamas gunmen on southern Israel on 7 October, in which about 1,300 people were killed and about 250 others taken hostage.
More than 25,700 people have been killed in Gaza since then, according to the territory’s Hamas-run health ministry.
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An estimated 1.7 million people – nearly three-quarters of the population – have also been displaced by the past 12 weeks of fighting and many of them are sheltering inside UN facilities or near them.
The Khan Younis Training Centre is one of the largest UNRWA shelters, with between 30,000 and 40,000 people said to be living inside its grounds.
UNRWA said the compound was clearly marked, that its co-ordinates had been shared with Israeli authorities, and that it and the civilians inside had to be protected under international law.
However, at least six displaced people were killed and many more injured when the training centre was struck on Monday during intense fighting in the surrounding area, according to the agency.
On Wednesday afternoon, UNRWA’s Gaza director, Thomas White, said a building at the facility that was housing 800 people from northern Gaza was hit.
In an interview with the BBC from Rafah in the evening, he said the building was struck by “two tank rounds” and that at least nine people were killed.
On Thursday morning, Mr White issued a statement saying the building was “hit by two shells and caught fire”. Twelve people had now been confirmed dead and 15 of the injured were in a critical condition, he added.
“A number of missions to assess the situation were denied. Yesterday evening, the UN finally managed to reach the affected areas to treat trauma patients, bring medical supplies and evacuate injured patients to Rafah.”
He added: “The situation in Khan Younis underscores a consistent failure to uphold the fundamental principles of international humanitarian law: distinction, proportionality and precautions in carrying out attacks. This is unacceptable and abhorrent and must stop.”
In response to UNRWA’s initial reports, the Israel Defense Forces (IDF) said: “After an examination of our operational systems, the IDF has currently ruled out that this incident is a result of an aerial or artillery strike by the IDF.”
“A thorough review of the operations of the forces in the vicinity is under way,” it added. “The IDF is also examining the possibility that the strike was a result of Hamas fire.”
Vedant Patel of the US state department repeated Washington’s calls for the protection of civilians in Gaza.
“We deplore today’s attack on the UN’s Khan Younis training centre,” she said, and called it “incredibly concerning”.
