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.