Israeli mortar rounds blasted a United Nations-run school that had been converted into a refugee shelter for hundreds of Palestinians displaced by the ten-day war in Gaza, killing more than 40 people.

It was one of three UN schools hit by Israeli ordnance yesterday. The strike against the Fakhora school in the northern town of Jabaliya was the deadliest single attack of an already blood-soaked offensive.

As international peace efforts gathered pace in the aftermath of the carnage, Ehud Olmert, the Israeli Prime Minister, backed down on his previous refusal to halt the offensive, and ordered the creation of a “humanitarian corridor” into the Gaza Strip. This would entail granting relief convoys periodic access to various areas of the territory to allow desperate Palestinians to stock up on vital goods, his office said last night.

Mark Regev, a spokesman for Mr Olmert, described the measure as a “special status to allow the transfer of people, foodstuffs and medicines” and said it could be implemented today.

Explaining the attack on the UN school, Israeli army officials said that their forces had been targeted by Hamas mortar fire from within the school compound. They named two alleged Hamas militants among the fatalities, Imad Abu Askar and Hassan Abu Askar. The Israeli Army has instructions to attack any position used by Hamas for firing rockets, an Israeli military source told The Times.

The deaths came as the Islamists were pushed back from their usual launch grounds to the east and into the packed metropolis of Gaza City and surrounding refugee camps. They pile more pressure on the international community to come up with a swift but durable formula to halt the offensive that has left more than 640 Palestinians dead.

President Sarkozy of France, who is on a peace mission, said that a deal to end Operation Cast Lead was close. Tony Blair, the international community’s envoy, said that a ceasefire could be reached within days but was contingent on finding a way to stop Hamas rearming. His hope that a truce could be struck was echoed by Gordon Brown, who warned that the Middle East faced its “darkest moment yet”.

The increasing violence forced Barack Obama, the US President-elect, to break his silence on Gaza. He expressed his “deep concern” at the killing of civilians.

The Fakhora school had been sheltering refugees driven from their homes by heavy fighting. Screaming relatives tried to revive victims who lay motionless in pools of blood on the pavement outside, as cars and ambulances rushed the casualties to Gaza’s already overwhelmed hospitals.

Palestinian medics said at least 42 bodies were pulled from the wreckage of the school that had been providing shelter for about 350 refugees. The UN said it had confirmed at least 30 dead and another 55 wounded.

John Ging, the Gaza director for the UN refugee agency, said that the building had been clearly marked with a UN flag and its precise co-ordinates given to the Israeli army to avoid just such a tragedy. The UN has run a number of schools in the Gaza Strip since the first wave of Palestinian refugees arrived more than 50 years ago.

Yesterday’s incident could prove a turning point in the operation. The death of 28 Lebanese civilians in an Israeli bombing of the village of Qana during the 2006 Lebanon war caused support for Israel to evaporate and hastened an end to the conflict. Israeli army officials said that UN schools had been used as firing positions by the Islamists before. “An initial inquiry by forces on the ground indicates that a number of mortar shells were apparently fired at [Israeli] troops from within the schools and the forces responded by firing a number of mortar shells into the area,” they said.

Chris Gunness, a spokesman for the UN Relief and Works Agency, called for a full investigation. “We want to see if there have been violations of international law,” he said.

Mr Ging said that the area had been the scene of a number of clashes between Israeli and Hamas forces, “so there’s an intense military and militant activity in that area”. But he said that his staff vetted Palestinians seeking shelter for militants.

In an earlier strike on another UN school in Beach Camp, on Gaza City’s Mediterranean coast, three cousins were killed. A third UN school was struck in Rafah in the south after Israeli tanks moved into the nearby city of Khan Younis. A building next to a UN health clinic was damaged during the Israelis’ relentless offensive to smash Hamas’s military capability before a ceasefire is forced upon them.

The mounting death toll intensified international efforts to secure a truce, with talk of an observers force being deployed. Mr Sarkozy, who met President Mubarak of Egypt in the latest round of his shuttle diplomacy, said: “I am confident that the Israeli authorities’ reaction will make it possible to consider putting an end to the operation . . . in Gaza.”

Mr Blair said that a key element to any truce would be for security measures to be implemented on Gaza’s border with Egypt, which has been used by Hamas as a resupply route. There are believed to be hundreds of smuggling tunnels under the border, although Israeli airstrikes are thought to have destroyed at least 40 large tunnels in the past week.

Diplomats are discussing a possible Turkish role in monitoring a ceasefire in Gaza. Talks at the UN are focusing on the creation of an international presence to oversee the ceasefire, reopen border crossings into Israel, and prevent arms smuggling from Egypt. Both France and Turkey signalled that they were ready to contribute to an international monitoring team for a ceasefire in Gaza.

Britain and the United States voiced support for the Egyptian peace initiative. “It’s very important the discussions we have here and any positive developments on the ground in the region are mutually reinforcing,” David Miliband, the Foreign Secretary, said at the UN in New York.

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