An airstrike on the Gaza Strip killed Hamas’s interior minister today as Israel stepped up its aerial bombardment of the territory.

Said Siam was staying at his brother’s house when it was destroyed by an Israeli missile killing both men and Mr Siam’s son.

Mr Siam is the most senior party official to have died in the 20-day onslaught, which has killed almost 1,100 people in Gaza. As interior minister he was in charge of the Hamas police and security force, many of whom are actively involved in fighting Israel.

He is thought to have belonged to the hard-line wing of Hamas and played a key role in communication between the political and military officials. Mr Said created the Executive Force, a militia that orchestrated the Islamist takeover of Gaza in June 2007.

A report on al-Quds television, a Hamas station based in Beirut, said: “Leader Said Siam, his son and his brother fell as martyrs in Gaza.”

Mohammed Nazzal, a Hamas leader in Damascus confirmed the report.

The Israeli Defence Forces said it had struck the home of Mr Siam’s brother north of Gaza City. Officials said forces attacked the house after receiving information he was inside.

“In a joint operation of the IDF and the Shin Beth [internal security] a short while ago, jets attacked a building,” a spokeswoman said. “We identified hitting the target."

Earlier today, the UN reported that its main compound in Gaza was struck by shelling that included white phosphorus, which is banned in civilian areas under the Geneva Convention because of the horrific burns injuries inflicted on victims.

The IDF insisted that all weapons used in Gaza were "within the scope of international law" but Ban Ki Moon, the UN Secretary-General, expressed his "strong protest and outrage" at the shelling and demanded an investigation.

Israeli leaders said their forces had been returning fire from within the UN compound but Gordon Brown said Israel’s shelling of the United Nations HQ was indefensible.

“When the United Nations is doing such vital work - humanitarian work amongst women and children in Gaza - no-one can defend this attack by Israeli forces on the compound,” the Prime Minister said. “The Israelis have now admitted it is a serious mistake. I have said to Ban Ki-moon this afternoon we will step up our efforts for an immediate ceasefire.”

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