Two White House officials are said to have illegally disclosed to journalists that Valerie Plame, the wife of former diplomat Joseph Wilson, was an agent for the Central Intelligence Agency.

It is alleged that this was an act of revenge because Mr Wilson had accused the Bush administration of exaggerating the case for war against Saddam Hussein.

Democratic presidential hopefuls Howard Dean and Wesley Clark said a special investigator should be appointed.

Although the Department of Justice is looking into the matter, some have expressed concerns that it could come under political pressure.

Mr Clark said an independent commission was needed.

"This administration has played politics with national security for a long time, but this is going too far," he told Reuters news agency.

Mr Dean called for a thorough investigation free from political pressure, suggesting it be carried out by the independent Department of Justice inspector general.

The White House has said it will co-operate with the Department of Justice by handing over telephone records and other information on request.

It has denied allegations linking Mr Bush's senior political adviser, Karl Rove, to the claim.

Before the war in Iraq, Mr Wilson was sent by the CIA to the West African state of Niger in order to investigate claims that Iraq had tried to buy nuclear material there.

His report concluded that there was no evidence for the claims.

Despite this, Mr Bush referred to them in his State of the Union address in January.

In a subsequent article in the New York Times, Mr Wilson questioned why his report had been ignored, forcing the White House to admit it had been a mistake to include the claim in the address.

The fact that Mr Wilson's wife worked for the CIA was published by a columnist, Robert Novak, shortly after Mr Wilson's article appeared. He has refused to divulge his source.

"I don't think people appreciate very much ... the unnecessary dragging of my wife's name into this sphere," Wilson told CNN.

The Washington Post has reported an allegation that White House officials blew the cover of Mr Wilson's wife in order to discredit him by suggesting he had been given the Niger mission only at his wife's urging. A leak of classified information is a federal crime punishable by up to 10 years in prison.

The White House on Monday rejected Democratic demands that an independent counsel be appointed to find out who leaked secret information apparently aimed at discrediting a vocal critic of prewar intelligence on Iraq.

The sudden squall over the leak that blew the cover of an undercover CIA operative energised Democratic presidential candidates and deepened tension over the administration's failure to find promised weapons of mass destruction in Iraq.

Officials said the Justice Department began a preliminary inquiry to determine if there should be a full-blown probe based on a memo from the CIA stating that a leak had occurred.

An official said one thrust of the inquiry would be to determine whether the leak violated the law or national security, or caused any damage.

White House spokesman Scott McClellan, during a lively news briefing, said no internal investigation was planned.

"At this point, I think the Department of Justice would be the appropriate one to look into a matter like this ... There are a lot of career professionals at the Department of Justice that address matters like this."

Retired Gen. Wesley Clark, a Democratic presidential hopeful, said an independent commission was needed.