National security adviser Condoleezza Rice commented on the claims from unnamed sources and Syrian dissidents on Friday.
"I want to be very clear: We don't, at this point, have any indications that I would consider credible and firm that that has taken place. But we will tie down every lead," said national security adviser Condoleezza Rice.
However, Rice refused to rule out the scenario raised by Paris-based Syrian dissident Nizar Nayyuf who told Britain's independent Channel Five News that a senior Syrian military intelligence source had told him about the weapons.
"There hasn't been any hard evidence that such a thing happened," Rice told reporters, but "I can't dismiss anything that we haven't had an opportunity to fully assess."
US-led forces in control of Iraq for months have yet to locate the alleged arsenals of chemical and biological weapons that President George Bush accused Saddam Hussein of possessing in violation of UN resolutions.
The unnamed source revealed that such weapons were smuggled across the Iraqi border in ambulances before the war that led to Saddam's ouster, and hidden at three sites in Syria, Nayyuf said.
Syria's al-Assad says Israel must
also renounce WMDs
"I knew this man during the last two years, he sent me much information," Nayyuf, a Syrian opposition activist and journalist, said of his contact.
He added Saddam moved the weapons last February and March because he knew he faced defeat at the hands of the US-led coalition.
"I don't think we are at the point that we can make a judgment on this issue. But obviously we're going to follow up every lead and it would be a serious problem if that, in fact, did happen," said Rice.
The Foreign Office in London gave a cautious response to Nayyuf's claims.
"If there is new information we would naturally follow it up," a spokesman said.
Syrian President Bashar al-Assad on Tuesday rejected British and US calls to renounce weapons of mass destruction and indicated he would not abandon his country's suspected chemical and biological programmes unless Israel gave up its undeclared nuclear arsenal.
"We are a country which is (partially) occupied, and from time to time we are exposed to Israeli aggression," al-Assad told the London-based Daily Telegraph newspaper.
"It is natural for us to look for means to defend ourselves," he said.