The United Nations has accused Israel of evacuating scores of Palestinians into a house in the suburbs of Gaza City, only to shell 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, including young children.

The Israeli military said it was investigating the claim but had no knowledge of the incident.

Citing "several testimonies" - but without identifying its sources - 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.

The ICRC declined to comment on the allegations in the OCHA report but a spokeswoman said that Red Cross teams had returned to Zeitun yesterday and evacuated around 100 people.

An Israeli human rights group, B’Tselem, quoted a Zeitun resident giving details of the alleged incident over the telephone but stressed that it was unable to independently verify the account.

Meysa Fawzi al Samuni, 19, said soldiers forced her and dozens of others to move into the warehouse-like home of another resident. Two men who left the house to pick up a relative were struck by "a missile or a shell," she said.

"My husband went over to them to help, and then a shell or missile was fired onto the roof of the warehouse. Based on the intensity of the strike, I think it was a missile from an F-16," B’Tselem quoted her as saying.

"After the smoke and dust cleared a bit, I looked around and saw 20-30 people who were dead, and about 20 who were wounded. As far as I know, the dead and wounded who were under the ruins are still there."

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 around 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.

After a meeting of his security cabinet, Ehud Olmert, the Israeli Prime Minister, rejected the resolution as "unworkable" today and said that Israel had "never agreed to let an external body decide its right to protect the security of its citizens".

"The firing of rockets this morning only goes to show that the UN decision is unworkable and will not be adhered to by the murderous Palestinian organisations," he added.

Hamas, the Islamic militant group which controls the Gaza Strip, also rejected the UN resolution.

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