The White House has strongly denied claims that it illegally leaked to the press the identity of an undercover CIA agent.
No evidence of weapons of mass destruction have yet been found
A spokesman for President George W Bush said the allegations were "simply not true".
A report in the Washington Post has alleged that the wife of former ambassador Joe Wilson was revealed as a CIA agent in retaliation for a report by Mr Wilson which scotched the administration's claims about Iraq's nuclear ambitions.
Several opposition Democrats are calling for an independent inquiry into the matter.
The BBC's Justin Webb in Washington says the allegation, if proved, could be highly damaging to the president and his senior officials.
The Department of Justice is investigating the claim at the request of the CIA. But several Democrats, including presidential hopefuls Joseph Lieberman and John Kerry, have expressed fears that the department could come under political pressure.
"I don't think we can leave this to the administration's own Justice Department," said Dick Gephardt, another of the 10 Democrats seeking to run as the party's candidate next year.
Ambassador Wilson was sent to the West African state of Niger to investigate claims that Saddam Hussein was trying to buy uranium there to build nuclear weapons.
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.
The Washington Post reported an administration official as saying that White House officials blew the cover of Mr Wilson's wife as punishment for his stance over the Niger issue.
The official told the newspaper the information was leaked to selected journalists in order to discredit the former diplomat by suggesting that he had been given the Niger mission only at his wife's urging.
The columnist Robert Novak broke the story in an article on 14 July citing "two senior administration officials" naming Mr Wilson's wife as Valerie Plame and saying she was an operative working on weapons proliferation.
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.
Mr Wilson said that, if true, the White House's actions were a "dastardly" and "contemptible" deed designed to intimidate people against speaking out.
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.
It is a serious criminal offence in the United States to make public the name of an agent.