State Department spokesman James Rubin told reporters: "I think it's fair to say that we have no reason to believe there have been significant efforts to reconstitute their weapons of mass destruction programmes."

But he argued that United Nations weapons inspectors should return.

No weapons inspectors have been in Iraq since last December, when the US and UK launched air strikes, accusing Baghdad of not co-operating with the UN arms commission.

Inspectors destroy Sarin-filled shells in 1998 in Iraq

"We need expert UN weapons inspectors on the ground with full Iraqi co-operation, as required under Security Council resolutions, for us to have high confidence that there is credible arms control in Iraq," Mr Rubin said.

The UN Security Council is divided over proposals to lift sanctions and restart relations with Iraq.

The US insists that Iraq must answer questions about its weapons programmes and adhere to strict controls on its oil revenue in any new UN plan.

Lab for decontamination

December's weapons inspection row was brought back into the spotlight on Thursday when a team of international experts began work in Baghdad to assess chemicals left behind when Unscom evacuated Iraq.

Their task is to remove toxic substances and dismantle the inspectors' laboratory.

A variety of chemicals, including mustard gas, remained in the Unscom facility in the Canal Hotel when the inspectors left in December.

Concerns about the safety of the laboratory were raised in the UN Security Council by Russia.

Iraq gave permission last month for the mission to enter the country.

The experts - accompanied by diplomats from Russia, France and China - arrived in Baghdad overland from Amman on Wednesday.

No one from the Unscom inspectorate is on the mission.

'No danger' in operation

The UN's representative in Iraq, Prakash Shah, said there was no hazard to either the experts or people in the area.

Iraq says the laboratory contains explosives which could blow up any moment.

Unscom believes at least 1kg of Iraqi-made mustard gas was left in the laboratories.