Powell also said he hoped a commission investigating the US intelligence on Iraq's alleged weapons of mass destruction would reveal how the CIA ended up depending on unreliable sources for key evidence he used to argue for war.

The acknowledgement about alleged mobile chemical arms laboratories could further hurt the credibility of the Bush administration, also under fire in an election year for failing to stop the September 11 attacks.

The United States justified its first preemptive war by accusing Iraq of amassing illegal arms and invaded last year without explicit UN approval and over the objections of many allies.

In February, 2003, Powell made a major presentation of the US case against Iraq at a special session of the UN Security Council, where he said the United States had several sources showing mobile chemical weapons laboratories.

But on Friday, the top American diplomat said the evidence on the trailers has been shown to be shaky.

"Now it appears not to be the case that it was that solid. But at the time I was preparing that presentation it was presented to me as solid," Powell told reporters on a flight home from a trip to Europe.

While doubts about the US sources of evidence for the laboratories have been raised for over a year, Powell's remarks were the most straightforward acknowledgement from the Bush administration that the information was probably wrong.

"That was the most dramatic of them (pieces of evidence) and I made sure it was multi-sourced," he said. "Now if the sources fell apart, then we need to find out how we've gotten ourselves in that position."

"I hope (the commission) will look into these matters to see whether or not the intelligence agency had a basis for the confidence that they placed in the intelligence at that time," he said.