The Israeli army on Sunday confirmed firing a mortar round into a Gaza UN shelter where 15 people died on Thursday, but denied killing anyone at the site.
Briefing journalists on the findings of an internal military enquiry into the incident at a UN school in Beit Hanun, spokesman Lieutenant Colonel Peter Lerner said militants "in the vicinity" of the school fired mortar rounds and anti-tank rockets at Israeli forces.
The army responded with mortar fire, sending a stray round into the compound.
"A single errant mortar (round) landed in the courtyard of the school," he said. "The courtyard was completely empty" at the time of the incident, he added.
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"We reject the claims that were made by various officials immediately following the incident, that people were killed in the school premises as a result of (Israeli army) operational activity," he added.
An AFP photographer who went to the scene saw blood spattered on the ground and Gaza emergency services spokesman Ashraf al-Qudra said 15 people died in the blast and at least another 200 people were injured.
"Many have been killed -- including women and children," UN Secretary General Ban Ki-moon said in a statement, adding that he was "appalled".
Lerner suggested the victims may have been hit in fighting raging elsewhere and "brought to the compound after injury" for first aid or shelter.