The UN today halted all aid supplies to Gaza amid a growing row over Israel's treatment of civilians in the area.
The agency said its workers were not safe after one of its supply truck drivers was killed by Israeli tank fire.
At the same time the Red Cross accused soldiers of a "shocking" disregard for wounded civilians and revealed four small children had been trapped for days beside the bodies of their dead mothers in their home.
A UN spokesman said: "The UN is suspending its aid operations in Gaza until we can get safety and security guarantees for our staff.
"We've been co-ordinating with them (Israeli forces) and yet our staff continue to be hit and killed."
He said the agency arranged the delivery with Israel, and the vehicle was marked with a UN flag and insignia when it was shot in northern Gaza. The Israeli army said it was investigating.
The UN has already demanded an investigation into Israel's shelling of one of its schools in Gaza that killed nearly 40 people earlier this week. Israel said militants were operating in the area.
The UN provides food aid to around 750,000 Gaza residents, and runs dozens of schools and clinics throughout the territory. They have some 9,000 locally-employed staff inside Gaza, and a small team of international staff who work there.
The Red Cross accusations came from officials at its HQ in Geneva who said the Israeli army refused them permission to reach a site in the Zaytun district for four days. Ambulances could not get through because the Israeli army had erected large earth barriers to block access.
When rescuers finally got through they "found four small children next to their dead mothers in one of the houses. They were too weak to stand up on their own. One man was also found alive, too weak to stand up," the Red Cross said. "In all, there were at least 12 corpses lying on mattresses", it added.
Meanwhile Israel faced the prospect of a second front after militants in Lebanon fired at least three rockets into the territory.
The attack raised the spectre of renewed hostilities on Israel's northern frontier, less than three years after the Hezbollah guerrilla group fought it to a standstill there.
One of the rockets went through the roof of a retirement home in Nahariya, about five miles from the border, and exploded in the kitchen as some 25 residents were eating breakfast in the adjacent dining hall. One resident suffered a broken leg, another bruises, apparently from slipping on the floor after emergency sprinklers came on.
Israel has repeatedly said it was prepared for a possible attack on the north since it launched its campaign against Hamas militants in Gaza, to the south.
In other Gaza violence, Israel killed at least 11 people, including three who were fleeing their homes, raising the death toll from its 13-day offensive to 699 Palestinians. Eleven Israelis have died since the offensive began on December 27.
The campaign is meant to halt years of Palestinian rocket attacks on southern Israel, but with roughly half the dead believed to be civilians, international efforts to broker a ceasefire have been gaining steam.
Although the UN Security Council failed to reach agreement on a cease-fire resolution, Egypt's UN Ambassador Maged Abdelaziz said representatives of Israel, Hamas and the Palestinian Authority agreed to meet separately with Egyptian officials in Cairo.
For Israel to accept a proposed ceasefire deal, "there has to be a total and complete cessation of all hostile fire from Gaza into Israel, and ... we have to see an arms embargo on Hamas that will receive international support," said a government spokesman.
Hamas said it would not accept a truce deal unless it includes an end to the Israeli blockade of Gaza - something Israel says it is not willing to do.