The UN Secretary General Ban Ki-moon has strongly condemned a third deadly attack on a UN school in Gaza, saying Israel was "repeatedly informed of the location of these sites".
The UN chief described Sunday's apparent Israeli airstrike on the school in Rafah as a "moral outrage and a criminal act".
He said it was a "gross violation of international humanitarian law," and called for those responsible to be held accountable.
At least ten civilians were killed and 30 wounded in the attack, the second in less than a week on a UN-run school sheltering civilians.
Palestinians carry an injured man after an Israeli strike on a UN school
At least 15 civilians died in a strike in Jabalya on Wednesday, days after 19 people died at a UN-run school in Beit Hanoun.
The Israeli army has said it is investigating the latest attack.
Robert Serry, the UN's Middle East Special Coordinator, confirmed the school was housing 3,000 displaced people.
"It is simply intolerable that another school has come under fire while designated to provide shelter for civilians fleeing the hostilities," he said.
The attack came as the total Palestinian death toll rose to more than 1,770, with at least 30 people killed in multiple strikes on Sunday.
Quadruplets born in Gaza this week
Israel, meanwhile, held a funeral for Second Lieutenant Hadar Goldin, who it reported missing on Friday.
Israel initially said it feared he had been kidnapped by Hamas militants, although it has since confirmed he died in combat.
"A special committee led by the Israel Defence Forces Chief Rabbi, announced the death of the IDF infantry officer of the Givati Brigade, Lieutenant Hadar Goldin, who was killed in battle in the Gaza Strip on Friday, August 1, 2014," an army statement said.
Confirmation of the soldier's death follows Israeli Prime Minister Benjamin Netanyahu's warning to Hamas that he is prepared to continue the operation in Gaza for as long as it takes to return his citizens to safety.
Troops in an Israeli tank give a thumbs up after withdrawing from Gaza
"We do not accept a continuation of the shooting," he told reporters, referring to ongoing Hamas rocket attacks.
"It (Hamas) will have to understand, however long that takes, that it will pay an intolerable price, from its perspective, for continuation of the shooting."
Meanwhile, a set of quadruplets born in Gaza on Wednesday have been discharged from hospital - but the shelling means they cannot return to their family home.
Grandfather Mefleh al Arjah said: "We live in Jenah but when the airstrikes and shelling started we fled to Tal Sultan, staying with extended relatives of our son, Ali. We left everything behind staying with them. We had nowhere else to go.
"We hope the war will end soon so we can return to our home."
Israel launched its aerial offensive on July 8 with the declared aim of ending "persistent" rocket fire by militants.
It subsequently sent in ground troops, shifting the focus of the operation to the destruction of a complex system of cross-border tunnels.
Israeli military officials have reported that 31 tunnels have been destroyed, fuelling speculation the offensive could soon wind down. Tanks have been seen returning to Israel and Palestinians in northern Gaza were told this weekend they could return to their homes.
Lieutenant-Colonel Peter Lerner said: "We are releasing troops from the front line but the mission is ongoing. Ground forces are operating. Air forces are operating."
On the Israeli side, 64 soldiers and three civilians have lost their lives.