US Secretary of State Colin Powell has conceded that Iraq may not have possessed any stocks of weapons of mass destruction before the war last year.
Powell said it was an open question whether WMDs existed in Iraq
His comments came after the former head of the US weapons inspection team, David Kay, said he did not believe there were any weapons stockpiles.
Mr Powell was speaking on his way to the former Soviet republic of Georgia.
Less than a year ago, Mr Powell warned the United Nations Security Council about the danger from Iraq's weapons.
In the run-up to the US-led war against Iraq, he gave a presentation to the Security Council, in which he asserted that Saddam Hussein had amassed secret weapons of mass destruction.
He said then that he believed Iraq possessed, among other things, between 100 and 500 tonnes of chemical weapons agents.
But in his latest remarks, he told reporters travelling with him that it was an "open question" whether Iraq had any stocks of weapons of mass destruction at all.
"The answer to that question is, we don't know yet,"
Mr Powell said on his way to attend the inauguration on Sunday of the new Georgian President, Mikhail Saakashvili.
On Friday, Mr Kay, who had led the US hunt for weapons in Iraq, resigned.
He told Reuters news agency he did not believe there had been large-scale production of chemical or biological weapons in Iraq since the end of the first Gulf War in 1991.
"I don't think they existed," Mr Kay said.
"What everyone was talking about is stockpiles produced after the end of the last Gulf War and I don't think there was a large-scale production programme in the 90s."
UK Prime Minister Tony Blair said he still believed the intelligence reports that Saddam Hussein's regime was developing weapons of mass destruction that he used to justify committing British troops to the war.
"I believe the intelligence was correct, and I think in the end we will have an explanation," he told the Observer newspaper in remarks published on Sunday but made before Mr Kay's statement.
"I have absolutely no doubt at all in my mind that the intelligence was genuine," Mr Blair added.
Responding to questions about Mr Kay's comments, Mr Powell said it was for the weapons inspectors still in Iraq to decide if there were any weapons stocks or not, where they had gone if they had existed, and, if there were ever any weapons, why that was not known before the war.
Mr Powell acknowledged that the US thought Saddam Hussein had illegal weapons, but added: "We had questions that needed to be answered.
"What was it?" he asked. "One hundred tonnes, 500 tonnes or zero tonnes? Was it so many litres of anthrax, 10 times that amount or nothing?"
The BBC's Jon Leyne, who is travelling with Mr Powell, says the secretary of state has made a significant concession on the weapons issue.
He says Mr Powell's language was very different from that of Vice-President Dick Cheney, who said just two days ago that it was too early to pass judgement on whether weapons of mass destruction existed.
Our correspondent says that with members of the Bush administration steadily backtracking from their earlier claims, the hunt for Iraq's weapons of mass destruction could have a very uncertain future once sovereignty is handed back to the Iraqis at the end of June.
Mr Kay has been replaced by Charles Duelfer, a 51-year-old former UN weapons inspector, who said he would not "pre-judge" the investigation despite previously saying that he did not believe banned weapons would be found.