Hans Blix said there was no evidence of mobile biological weapons laboratories or of Iraq trying to foil inspectors by moving equipment before his teams arrived.

In a series of leaks or previews, the state department has said Mr Powell will allege that Iraq moved mobile biological weapons laboratories ahead of an inspection. Dr Blix said he had already inspected two alleged mobile labs and found nothing: "Two food-testing trucks have been inspected and nothing has been found."

Dr Blix said that the problem of bio-weapons laboratories on trucks had been around for a while and that he had received tips from the US that led him to inspect trucks in Iraq. The Iraqis claimed that the trucks were used to inspect the quality of food production.

He also contested the theory that the Iraqis knew in advance what sites were to be inspected. He added that they expected to be bugged "by several nations" and took great care not to say anything Iraqis could overhear.

He said he assumed the US secretary of state would not be indicating sites that the inspectors should visit that he had not told them about. "It is more likely to be based upon satellite imagery and upon intercepts of telephone conversations or knowledge about Iraqi procurement of technical material or chemicals," he said.

Dr Blix is travelling to Baghdad for further meetings with Iraqi officials before reporting to the security council on February 14 and March 1.

He said the choice for the UN was between continued containment and invasion. Both strategies had problems, but an invasion required 250,000 troops and over $100bn while for containment the numbers were 250 inspectors and $80m.