The United Nations Relief and Works Agency says Israel's actions in shelling a UN compound in Gaza City last night should be investigated as a possible war crime. Israel's military carried out 50 air strikes as its cabinet prepared to vote on a proposal for a unilateral ceasefire.
A tank shell hit the UN school in northern Gaza, killing at least two people sheltering there. Israel says it is checking the report.
UN chief Ban Ki-moon says he has expressed "strong protest and outrage" to Israel over the shelling.
“I conveyed my strong protest and outrage to the defense minister and the foreign minister and demanded a full explanation," Ban said, adding that he had demanded an investigation into the shelling.
"The defense minister said to me it was a grave mistake and he took it very seriously. He assured me that extra attention will be paid to UN facilities and staff and this will not be repeated."
On Friday 16 January the United Nations security council passed a unanimous resolution on a cessation of hostilities. Israel's security cabinet will vote on Saturday evening on whether to declare a ceasefire, after its government said it thought "significant progress" had been made at talks with Egyptian mediators in Cairo.
Under the Egyptian proposal, fighting would stop immediately for 10 days and Israeli forces would remain in Gaza while the border crossings into the territory would remain closed. Egypt will also pressurise Hamas on rocket attacks.
But Hamas said it would ignore any truce if its conditions were not met. A spokesperson, Osama Abu Hemdan, told AFP news agency: "As long as it [the Israeli military] remains in Gaza, resistance and confrontation will continue."
In Washington DC, USA, Israeli Foreign Minister Tzipi Livni said Israel would have to suspend any ceasefire if Hamas continued to fire rockets.
Israeli warplanes renewed bombing raids on Gaza just before dawn on this morning, with heavy explosions south of Gaza City.
United Nations officials say two children, aged five and seven, were killed when Israeli tank fire hit a UN school where hundreds had taken shelter in the northern town of Beit Lahiya.
A spokesman for Unrwa in Gaza, Chris Gunness, said: "There has to be an investigation to determine whether a war crime has been committed."
This is not the first time the UN has talked about war crimes in Gaza. Earlier this month, UN High Commissioner for Human Rights Navi Pillay said some Israeli actions reported in Gaza might warrant prosecutions for war crimes.
1,193 people have been killed, including 410 children and 108 women since the conflict began on 27 December 2008. There have been 5,300 people wounded, including 1,600 children.
Thirteen Israelis, mostly soldiers, have been killed during the campaign.