Investigators planned on Friday to begin interviewing officials in President George Bush's administration in the coming days.

Separately, the State Department and Defence Department also confirmed they have been directed to preserve records linked to the widening probe into the disclosure.

At issue is who told a reporter that former ambassador Joseph Wilson's wife was a CIA agent, after the diplomat charged publicly that the case for war with Iraq was exaggerated.

Investigation

Opposition Democrats have called for congressional hearings and an independent investigation - appeals which Bush aides have categorically rejected.

"The president has directed everyone to cooperate fully with the Department of Justice. We want to get to the bottom of this, the sooner the better," presidential spokesman Scott McClellan said. 

Presidential advisor Karl Rove
has been linked to the leak 

The journalist who published the leak, Robert Novak, identified "two senior administration officials" as his sources.

Wilson has linked Bush's political guru, Karl Rove, to the leak. McClellan has called that "ridiculous".

CIA agent 

White House aides have until 21:00 GMT on Tuesday to hand over copies of any relevant materials, including emails, telephone logs and notes.

The Justice Department's latest demand follows a directive for all White House staff to safeguard materials linked to the leak of Valerie Plame's identity and her ties to the Central Intelligence Agency. 

The request seeks all documents tied to Wilson's February 2002 trip to Niger, where he discredited the charge Saddam Hussein sought uranium from the African nation, as well as materials about Plame's CIA activities.

A Washington Post/ABC News poll published on Thursday found that 69% of those surveyed believed an independent probe was needed, compared with only 29% who would leave it to Attorney General John Ashcroft.