Emergency workers attempt to salvage food aid while putting out a fire at the headquarters of the United Nations in Gaza City, which was set ablaze after coming under intense shelling from Israeli forces on Thursday. (Mohamed Al-Zanon, Maan Images)

Israeli tanks thrust deep inside Gaza City last night as ferocious fighting raged in dense residential areas with terrified families fleeing along streets echoing with gunfire, although many others were trapped in their homes.

Israeli shelling set fire to the UN headquarters, a hospital, a school and a building used by the media, leading to widespread international protests and renewed calls for a ceasefire in the conflict which has so far cost 1,073 Palestinian and 13 Israeli lives.

A senior Hamas leader, the Interior Minister Said Seyyam who was responsible for thousands of security agents, was killed in an Israeli raid which flattened his brother's home.

Three members of UN staff were injured when three Israeli shells hit the headquarters, setting it on fire.

Thousands of tonnes of desperately needed food and humanitarian supplies were destroyed and about 700 refugees given shelter in the building had to be evacuated.

UN officials said the shells were white phosphorus, believed to have been responsible for burns suffered by some Palestinian civilians.

The UN secretary general, Ban Ki-moon, on a peace mission to the region, said: "I conveyed my strong protest and outrage to the Defence Minister and Foreign Minister and demanded a full explanation."

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The Israeli Prime Minister, Ehud Olmert, apologised for the shelling but claimed Hamas fighters had opened fire from the centre. "It is absolutely true that we were attacked from that place, but the consequences are very sad and we apologise for it," he said. "I don't think it should have happened and I'm very sorry."

John Ging, the director of operations for the UN relief agency, UNRWA, in Gaza described the Israeli claim about a Hamas presence as "nonsense".

He added: "It's a total disaster for us." Mr Ging said the UN had warned the Israelis the compound was in danger from shelling that had begun overnight, and provided them with GPS co-ordinates to prevent an attack.

The Al-Quds hospital was also hit by shellfire when Israeli tanks moved further into the city. A tower housing the Reuters agency and other media outlets was also hit.

To read the full article, please visit The Independent.