A massive truck bomb devastated the UN headquarters in Baghdad yesterday, killing at least 16 people including the organisation's top official in Iraq.

Sergio Vieira de Mello, the special envoy of UN secretary general Kofi Annan, was buried under concrete and died several hours later after rescuers desperately tried to reach him in the rubble of his office.

At least 100 other people were injured in the attack in which a suicide bomber drove a yellow cement truck packed with explosives into the three-storey building, causing part of it to collapse.

US soldiers were still digging into the wreckage last night looking for trapped UN workers. The death total was expected to rise further.

The force of the explosion was seen on television screens around the world as a news conference was being held in the building at the time by the director of the UN's Oil for Food programme, Benon Sevan.

Cameras kept rolling and showed bloody journalists and officials covered in dust and debris.

The truck is believed to have approached from an alley at the rear, exploding just below Mr de Mello's office, and leaving a 50ft crater.

The bombing was condemned as an "outrage" by Foreign Secretary Jack Straw, who said it would only serve to strengthen the resolve of coalition forces in Iraq.

He also said it underlined that the decision to take military steps against Saddam Hussein had been right.

"It is one more reason why the action taken to free the Iraqi people from this kind of situation was correct," he said.

Prime Minister Tony Blair, who is on holiday in Barbados, said: "The perpetrators of the atrocity have demonstrated pure cowardice and are enemies not only of the UN and coalition but also of the Iraqi people."

Mr de Mello, a 55-year-old Brazilian, was the UN High Commissioner for Human Rights and arrived in Baghdad in June as Kofi Annan's Special Representative to the country.

He was trying to restore security, help Iraqis draft a constitution and move towards a democratically-elected government and was known as one of the UN's most tireless and forceful operators.

In the aftermath of the explosion, rescuers could see Mr de Mello alive and conscious in the rubble and tried desperately to reach him, pouring water over him to sustain him.

The building was home to most UN agencies and had been a base for weapons inspectors before the war.

It was not clear whether Iraqis loyal to Saddam or foreign Islamic fundamentalist terrorists were behind the attack.

Like the truck bomb attack at the Jordanian embassy in Baghdad two weeks ago, which killed 17 people, there was no claim of responsibility.

Suspicion was focusing on terror groups because the suicide bomb was a departure from Iraqi tactics which have so far involved unsophisticated hit-and-run gun attacks and explosives triggered by remote control.

Al Qaida views the UN as an ally of the US and the attack bore the hallmarks of one of its attacks.

The explosion blew out windows a mile away.

The UN Security Council condemned the bombing as a "terrorist criminal attack".