The death of forty-two people in an Israeli attack at a UN-run school in Gaza today finally forced Barack Obama to break his silence over the conflict.

"The loss of civilian life in Gaza and Israel is a source of deep concern for me," the US President-elect said.

He would not be drawn further, promising to abide by his principle that only George W. Bush would speak for American foreign policy while he was still President of the United States. Mr Obama added that would have plenty more to say after his inauguration on January 20.

Gordon Brown said that the recent fighting marked the "the darkest moment yet for the Middle East".

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.

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 US Administration appeared to be split on its stance towards the conflict. The State Department today made its first call for an end to the violence but a spokeswoman for the White House said it was confident that Israel was taking care to avoid civilian casualties in Gaza.

"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.

Dana Perino, spokeswoman for the White House, said: “I saw the reports about the school. I don’t have any information about that. I think that we should not jump to conclusions and we should wait to find out what the evidence says.

“What we do know is that Hamas often hides amongst innocents and uses innocent people, including children, as human shields."

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

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