Interviewed on US television, Mr Bremer said the blast that killed at least 20 people was "of a size not seen before".

The bomb is believed to have used more than 1,000lb of explosive - four times as much as in the attack on the Jordanian embassy two weeks ago.

The two bombings are "not yet connected", but both were "acts of disciplined people", Mr Bremer told the ABC channel.

Truck bombs have been used by militant groups, including Osama bin Laden's al-Qaida network, though Saddam's secret service is said to have trained in the tactic too.

According to reports by Associated Press, the explosive used against the UN building was C4, which has no non-military uses. C4 was used in al-Qaida's attack on the USS Cole in Aden harbour, Yemen, three years ago.

Tuesday's choice of target gives no clue as to the culprit: all the suspects are likely to regard the UN more as a collaborator in Iraq's occupation than a bringer of aid.

Attacking the UN and other soft targets - such as the Jordanian embassy and pipelines carrying oil or water - also puts the US in difficulty by signalling that it has too few troops to maintain security.

In addition, it can help to convince ordinary Iraqis that chaos will continue until the US leaves.

A suicide attack is more likely to indicate Islamist militants than Baathists, though Mr Bremer said it was still unclear whether the UN bombing was suicide or not. The earlier explosion at the Jordanian embassy was triggered by remote control.

Ahmad Chalabi, a member of the governing council, who previously led the opposition Iraqi National Congress from exile, claimed yesterday that Baathists and Islamist militants had been jointly planning a major attack.

He told a news conference in Baghdad that on August 14 the governing council received intelligence that Saddam supporters and militants had met to discuss attacking an Iraqi political party or the UN.

"It specifically said that this attack would take place using a truck to be detonated either through a suicide mechanism or through electronic detonation," he said, adding that the information had been passed on to the Americans.