When the Israeli attack came on the Prep C girls school in the Jabaliya camp at 3.45pm on Tuesday, Mr Ging said about 350 Palestinians were inside, using the school as a shelter. "The reason they were in the school is because the Israelis had told them to go there," he said. "They were doing exactly as they had been told."

To ensure there was no confusion about the school's location, Mr Ging said the United Nations gave specific GPS co-ordinates to Israeli Defence Forces commanders. But after receiving reports that mortar shells had been fired at Israeli units from within the school grounds, the commanders decided to strike back. "They shot back to save their own lives," said Israeli Defence Forces spokesman Ilan Tal. Three tanks shells exploded just outside the school grounds, causing pandemonium inside.

"As you can imagine, this was a very distressing attack," Mr Ging said. "It has killed 40 people, and some of those who were injured may yet die. "Now we need to know how this happened. A full, transparent and independent inquiry has to be held to determine exactly how this occurred."

The Israeli Defence Forces maintained that mortars had been fired from the school. In a statement released by spokesman Captain Benjamin Rutland, it said it had incontrovertible evidence to support its version of events. "This is not the first time that Hamas has fired mortars and rockets from schools, in such a way deliberately using civilians as human shields in their acts of terror against Israel," the statement said." Brigadier-General Avi Benayahu said the Israeli Defence Forces had established that Hamas terror operatives were among the dead at the school, and had proof of a mortar battery cell that had been firing in the area. "Hamas operatives Imad Abu Askhar and Hassan Abu Askhar were among terrorists that were identified to be killed," he said. The attack on the school has caused widespread shock across Israel, intensifying pressure on Prime Minister Ehud Olmert to reach a ceasefire.

After 13 days of bombardment of Gaza from the air, land and sea, which has claimed more than 660 lives, the past few days of fighting have been among the bloodiest in the history of the Israel-Palestinian conflict. Five Israeli soldiers lost their lives on Wednesday, four from friendly fire, bringing to 10 the number of Israelis who have been killed. Just before midnight on Tuesday, three Gazans were killed when Israeli forces attacked another UN school in Gaza City.

Earlier that day, a shell hit a house in the Beach refugee camp, killing at least seven members of one family. Shelling of houses in the Bureij refugee camp left at least five dead and 16 injured.