The Israeli army has insisted that a mortar round it fired into a UN shelter in Gaza on Thursday in a clash with "militants" was not responsible for the deaths of at least 15 people.
The Israel Defense Forces (IDF) said in a statement that only a "single errant mortar" landed in the school courtyard.
It has released aerial footage which it says proves the shelter was empty at the time the stray shell went off.
UN chief Ban Ki-moon said last week that he was "appalled" by the attack.
Palestinian officials say that three Israeli tank shells hit the school in the town of Beit Hanoun in one of the single bloodiest incidents during three weeks of Israel-Hamas fighting. At least 200 sheltering Palestinians were injured, they added.
But Israel says that at the time the round was fired the IDF was itself under attack by mortar fire and anti-tank rockets.
Israeli army spokesman Peter Lerner said that the courtyard of the school "was completely empty" at the time of the incident.
"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 said.
Lt-Col Lerner suggested that the victims may have been "brought to the compound after injury" for medical treatment or shelter after being caught in a crossfire between Israeli troops and Gaza militants.
He used aerial footage released on Sunday that, he said, proved the shelter's courtyard was empty at the time the mortar was fired.
The United Nations Relief and Works Agency (UNRWA) that operates the school has called for a full investigation.
UNRWA spokesman Chris Gunness said on Friday that Israel had been given the co-ordinates of the building in Beit Hanoun and informed that it was being used to shelter Palestinian families fleeing the offensive against Hamas militants.
UN officials said that during the course of Thursday they had been trying to negotiate a window of time with the Israeli army for civilians to leave the area because of heavy fighting.
Pools of dried blood can be seen on the ground in the courtyard of the school as well as a large scorch mark which appears to have been made by a shell.
The IDF statement added that it does not "operate or target international organisations in the Gaza Strip".
Israel launched its military offensive on 8 July with the declared objective of stopping Hamas firing rockets from Gaza.
According to the UN, more than 118,000 people are now sheltering in UN schools and people are running out of food.