The United Nations headquarters in Baghdad has been destroyed by a truck bomb killing 20 people in one of the deadliest attacks ever directed at the United Nations.

Among the dead was Sergio Vieira de Mello, the top UN official in Baghdad. Dozens were also wounded.

The explosion happened at about 4:30 in the afternoon yesterday while hundreds of UN officials, workers and journalists were inside the converted hotel. A cement truck loaded with explosives is believed to have crashed into the building.

The attack came less than two weeks after a car bomb destroyed the Jordanian embassy in Baghdad killing 17.

No group took responsibility for the attacks. Paul Bremer, the who is overseeing the US. Occupation accused Syria of permitting militants into Iraq.

US and UN officials said de Mello may have been the target of the attack since the bomb struck so close to his office.