The United Nations has accused Israeli troops of evacuating scores of Palestinians – including children – into a house in Gaza and then shelling the property 24 hours later, killing some 30 people.

In a report published today on what it called "one of the gravest incidents" of the 14-day conflict, the UN's Office for the Coordination of Humanitarian Affairs (OCHA) complained that the Israeli Defence Force then prevented medical teams from entering the area to evacuate the wounded.

Citing "several testimonies" - but without identifying them - OCHA said that Israeli foot soldiers evacuated around 110 Palestinians into a house in Zeitun, south of Gaza City, on Sunday. Half of them were children.

"Twenty-four hours later, Israeli forces shelled the home repeatedly, killing approximately 30," OCHA said. "Those who survived and were able walked two kilometres to Salah Ed Din road before being transported to the hospital in civilian vehicles. Three children, the youngest of whom was five months old, died upon arrival at the hospital."

OCHA added that the International Committee of the Red Cross took several days to gain access to the area but managed to do so on Tuesday during the first daily three-hour humanitarian ceasefire. "Due to the limited time allowed, ICRC was not able to reach all houses in the area," it said. "In all, ICRC evacuated 30 Palestinians including 18 wounded."

The ICRC had already issued a statement on the Zeitun incident yesterday, accusing Israel of "unacceptable behaviour" and of breaching international humanitarian law. In that statement, the ICRC said its team had discovered four emaciated children living next to the corpses of their dead mothers in a house on which there were 12 dead bodies lying on mattresses.

In another house, they found 15 survivors of the Israeli bombardment, several of them wounded, and, in a third, three corpses.

An ICRC spokeswoman in Geneva said that she could not confirm that the report referred to the same incident. The ICRC statement had made no mention of Israeli shelling Palestinian evacuees.

Responding to the OCHA report, the Israeli army said it had no knowledge of such an incident but was investigating.

The allegations risk further souring relations between Israel and the United Nations. The main UN aid agency in Gaza, UNRWA, said yesterday that it was suspending operations after two of its drivers were killed in an Israeli attack on an aid convoy.

Late last night in New York, the UN Security Council passed a resolution demanding an immediate ceasefire in Gaza, where some 800 Palestinians have been killed in a two-week Israeli offensive.

The 15-member council backed a resolution drafted by Britain in a 14-0 vote after the United States lifted its veto threat - infuriating the Israelis - but decided nevertheless to abstain.

The resolution "stresses the urgency of and calls for an immediate, durable and fully respected ceasefire leading to the full withdrawal of Israeli forces from Gaza."

But the Israeli shelling continued unabated this morning, small plumes of white smoke rising every few minutes from the battered apartment blocks of Gaza City.

Israel carried out more than 50 air strikes in Gaza overnight, in which 12 Palestinian civilians were reported to have been killed. The Israeli military said that the militant group Hamas, which controls the Gaza Strip, and its allies fired more than 15 rockets into southern Israel, wounding one person.

Israel's security cabinet - comprising Ehud Olmert, the Prime Minister, Ehud Barak, the Defence Minister and the Foreign Minister Tzipi Livni - was to meet later today to consider a formal response the UN resolution.

But Ms Livni made Israel's position clear in a statement this morning in which she said: "Israel has acted, is acting and will act only according to its considerations, the security needs of its citizens and its right to self defence."

Hamas also rejected the ceasefire demand. "Even though we are the main actors on the ground in Gaza, we were not consulted about this resolution and they have not taken into account our vision and the interests of our people," one Hamas official said. "As a result we do not feel concerned by this resolution and when the different parties apply it they will have to deal with those who are in charge on the ground."

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