Forty-two women and children were killed in an attack at a UN-run school in Gaza today, the third such assault in 24 hours.

The attack, which appeared to come from an air strike from two Israeli F16s, happened at the al-Fakhora school in Jabaliya refugee camp, causing carnage inside and outside the building, where hundreds of Palestinians had sought refuge from fighting between Israeli soldiers and Hamas militants. Initial reports had said the devastation was caused by a tank shell exploding.

Two tank shells exploded outside the al-Fakhora school in Jabaliya refugee camp, spraying shrapnel on those inside and outside the building, where hundreds of Palestinians had sought refuge from fighting between Israeli soldiers and Hamas militants.

Bodies lay scattered on the ground in pools of blood amid shredded clothing and shoes after the attack, in which several dozen people were also wounded.

As the death toll in the conflict rose, the United States today made its first call for an end to the violence.

"We would like an immediate ceasefire, absolutely,” said Sean McCormack, a State Department spokesman. “An immediate ceasefire that is durable, sustainable and not time-limited."

Israel has continued to defy international calls for a ceasefire and earlier moved its troops deeper into southern Gaza to intensify its ground offensive.

Those who died in the attack today were people sheltering in the school and local residents, medical officials at two local hospitals said.

“I saw a lot of women and children wheeled in,” said Fares Ghanem, an official at Kamal Radwan Hospital. “A lot of the wounded were missing limbs and a lot of the dead were in pieces.”

Schools run by the UN Relief and Works Agency (UNRWA) have opened their doors to provide shelters for thousands of Palestinians who have fled their homes. At least 17 achools are heltering more than 5,000 people. Nine of the schools in the Jabaliya refugee camp are in the front line of fighting.

Three people were also killed at another UN-run school in the southern city of Khan Younis when Israeli tank fire showered the building where they were sheltering from the conflict. More than 600 Palestinians have lost their lives during the battle between Israeli forces and Hamas militants, which is now in its 11th day. A quarter of those are civilians, according to the UN. Three peoplealso died in an air strike on a school in the Shati refugee camp in Gaza City, medics said.

Max Gaylard, the UN humanitarian co-ordinator for the Palestinian territories, said that Israel had the GPS co-ordinates of all UN buildings in Gaza – including its schools. “Neither homes nor UN shelters are safe” for civilians, he said in a statement that reaffirmed UN ceasefire calls.

“These tragic incidents need to be investigated, and if international humanitarian law has been contravened, those responsible must be held accountable." .

John Ging, the top UN official in Gaza, said: "There's nowhere safe in Gaza. Everyone here is terrorised and traumatised."

Speaking from the largest hospital in the region, he said: "I am appealing to political leaders here and in the region and the world to get their act together and stop this. "They are responsible for these deaths."

Twenty-four Hamas rockets were fired into Israel today; one wounding an Israeli infant.

Five Israeli soldiers have been killed in the past 24 hours. One died in an attack by Hamas militants in the north of Gaza city this morning.

Four soldiers were killed and 30 wounded in two separate “friendly fire” incidents overnight. Three of them were killed when the building in which they were taking cover was hit by one of their own tanks. The fourth soldier was also killed by an errant tank shell. Israel has lost six soldiers since the start of ground incursions into Gaza on Saturday.

The Israelis today moved troops into Khan Younis in southern Gaza widening the ground assault that it began four days ago after a week of air strikes failed to stamp out cross-border rocket fire.

President Sarkozy of France has travelled to Syria and Lebanon to seek a deal for a cessation of violence amid international efforts to broker a ceasefire.

Ehud Olmert, the Prime Minister, said a ceasefire could be put in place only if Hamas were prevented from rearming and ended rocket attacks into Israel. "I have dedicated the past two years to reach a peace agreement with Syria and the Palestinians. I am not a man of war; I am a man of compromise. I am a man of peace, whose conditions of compromise may be painful, but will bring peace to the people of Israel," he said.

Tony Blair, the Middle East envoy for the Quartet of the EU, the United States, the UN and Russia, said that a ceasefire was achievable if the supply of arms to Hamas was blocked through the border between Egypt and Gaza.

Israeli forces said that they had struck the home of one of the founders of the Hamas rocket division this morning. They said Iman Siam was in his house at the time of the air strike in Jabalya, in northern Gaza.

The British Government has announced that it will send £4 million for humanitarian aid to the region, the first tranche of the £6.8 million it recently agreed to send to Gaza to help to meet basic needs such as food, water and fuel.