The White House says it will co-operate with an investigation by the US Justice Department over new allegations surrounding the case for war against Iraq.
No evidence of weapons of mass destruction have yet been found
The White House has strongly denied claims that it illegally leaked to the press the identity of an undercover CIA agent.
A spokesman for President George Bush said White House staff would hand over telephone records and other information if they were requested by the investigators.
Several opposition Democrats are calling for a wider, independent inquiry into the allegations, as with British inquiry into the death of the UK weapons scientist Dr David Kelly.
The BBC's Justin Webb in Washington says the allegation, if proved, could be highly damaging to the president and his senior officials. The leaking of the name of a serving CIA agent is a criminal offence.
It is alleged that two White House officials disclosed to journalists that the wife of a former diplomat, Joseph Wilson, was a CIA agent, after Mr Wilson publicly questioned some of the information used to justify the Iraq war.
A report in the Washington Post alleged that Valerie Plame was outed as a CIA agent in retaliation for a report by Mr Wilson which scotched the administration's claims about Iraq's nuclear ambitions.
Before the war, Mr Wilson was sent by the CIA to the West African state of Niger in order to investigate claims that Iraq has tried to buy nuclear material there.
His report said there was no evidence for the claims.
Despite this, Mr Bush referred to them in his State of the Nation speech in January.
Mr Wilson later questioned publicly why his report had been ignored, sparking a media outcry and causing the White House to admit it had been a mistake to include the claim in the address.
Democrats smell blood
The Washington Post has reported an administration official as saying that White House officials blew the cover of Mr Wilson's wife in order to discredit the former diplomat by suggesting that he had been given the Niger mission only at his wife's urging.
The Department of Justice is investigating the claim at the request of the CIA, but several opposition Democrats are calling for a special investigator to be appointed.
Democratic presidential hopefuls, including Howard Dean, Joseph Lieberman and John Kerry, have expressed fears that the justice department could come under political pressure.
The BBC's Matt Frei in Washington says whether the allegations are true or not, the Democrats are trying to make the most of them.
"What has gone on in this case is one of the most dastardly, despicable things that I have seen in my more than 20 years in Washington and speaks of how far some will go to stifle dissent," said New York Senator Charles Schumer.
Mr Wilson has said that four reporters had told him that White House officials had contacted them in July to encourage stories which would reveal his wife's identity.
White House spokesman Scott McClellan has emphatically denied the claims.
"It is simply not the way the White House operates," he said.
However, he said anyone found to have leaked the information "should be pursued to the fullest extent by the Department of Justice. The president expects everyone to adhere to the highest standards of conduct".
In particular, Mr McClellan denied allegations which linked Mr Bush's senior political advisor Karl Rove to the claim.
"The president knows he wasn't involved... It's simply not true," he said.
Mr Wilson has said that he believes Mr Rove at least condoned the revelation of Ms Plame's identity.