How The Times broke the story | Photographic evidence | Spent shells prove use | The burn victims | War in Gaza photojournalism

The main UN compound in Gaza was left in flames today after being struck by Israeli artillery fire, and a spokesman said that the building had been hit by shells containing the incendiary agent white phosphorus.

The attack on the headquarters of the UN Relief and Works Agency (UNRWA) came as Ban Ki Moon, the UN Secretary-General, arrived in Israel on a peace mission and plunged Israel's relations with the world body to a new low.

Mr Ban expressed his "strong protest and outrage" at the shelling and demanded an investigation, only to be told by apologetic Israeli leaders that their forces had been returning fire from within the UN compound.

"The Israeli forces were attacked from there and their response was severe," Ehud Olmert, the Prime Minister, told the UN chief, according to a statement released by his office.

"We do not want such incidents to take place and I am sorry for it but I don’t know if you know, but Hamas fired from the UNRWA site. This is a sad incident and I apologise for it."

UNWRA, which looks after around four million Palestinian refugees in the region, suspended its operations in Gaza after the attack, in which three of its employees were injured.

Chris Gunness, a UNRWA spokesman, said that the building had been used to shelter hundreds of people fleeing Israel’s 20-day offensive in Gaza. He said that pallets with supplies desperately needed by Palestinians in Gaza were on fire.

"What more stark symbolism do you need?" he said. "You can’t put out white phosphorus with traditional methods such as fire extinguishers. You need sand, we don’t have sand."

The Israeli military has denied using white phosphorus shells in the Gaza offensive, although an investigation by The Times has revealed that dozens of Palestinians in Gaza have sustained serious injuries from the substance, which burns at extremely high temperatures.

The Geneva Convention of 1980 proscribes the use of white phosphorus as a weapon of war in civilian areas, although it can be used to create a smokescreen. The Israel Defence Forces (IDF) said today that all weapons used in Gaza were "within the scope of international law".

The attack on the UN compound came as Israeli forces pushed deeper into Gaza City and unleashed their heaviest shelling on its crowded neighbourhoods in three weeks of war. At least 15 Palestinians were killed in the Israeli attacks, medical officials said, pushing the death toll up towards 1,100 — a level that Mr Ban described as "unbearable".

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