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The main UN compound in Gaza was 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 told reporters in Tel Aviv that he had expressed “strong protest and outrage” to the Israeli Government over the shelling of the compound and was demanding an investigation. He said that Ehud Barak, the Israeli Defence Minister, had told him that it was “a grave mistake”.

UNWRA, which looks after around four million Palestinian refugees in the region, suspended its operations in Gaza after the attack, in which it said that three of its employees had been 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”.

It was not clear whether the escalation signalled a new phase in the conflict. Israel has held back from all-out urban warfare in the narrow alleyways of Gaza’s cities, where Hamas militants are more familiar with the lay of the land.

Black smoke billowed over Gaza City, terrifying civilians who said that they had “nowhere left to hide” from the relentless shelling.