White House adviser Karl Rove spoke to a journalist about a CIA agent days before her identity was revealed in the press, US magazine Newsweek has said.
Karl Rove is one of President Bush's closest advisers
The disclosure of the agent's name - a possible crime - sparked a federal investigation that has become a battle between the government and the press.
Journalist Matthew Cooper has now agreed to testify, after Mr Rove apparently said he could do so.
Mr Rove has denied he was behind the disclosure of the agent's name.
Cooper, who writes for Time magazine, and fellow journalist Judith Miller of the New York Times, had both been ordered to testify about their sources in the leak case.
Both had refused to do so, sparking a debate in the US over the right of the press to keep certain contacts confidential.
Last Wednesday, Cooper said he would abandon his opposition to testifying after his source gave him permission at the last minute to reveal his identity.
Miller maintained her refusal to testify - arguing that it was her duty as a journalist to protect her sources - and was jailed.
She is expected to be in prison for up to three months until the court ends its investigation - or until she changes her mind.
Newsweek quoted Mr Rove's lawyer, Robert Luskin, as saying Mr Rove discussed the CIA agent with Cooper days before her identity was revealed by another journalist.
Mr Luskin was quoted as saying it was his client who gave Cooper permission to testify.
Prosecutors are investigating how the name of CIA agent Valerie Plame was released to the press in 2003.
The wilful disclosure of a covert CIA agent's name can be a federal offence.
According to Newsweek, Mr Rove did not reveal Ms Plame's name and was not aware she was a secret agent.
'No knowing disclosure'
Ms Plame's husband, former US ambassador Joseph Wilson, has alleged that his wife's identity was made public in an attempt to discredit him after he challenged the government's arguments for going to war in Iraq.
Mr Wilson says he travelled to Niger to investigate a claim that Iraq had tried to buy nuclear material there but found no evidence to prove it.
The claim was later used by US President George W Bush to justify invading Iraq.
US journalists have rallied in support of Miller and Cooper
The article in Newsweek quotes a confidential e-mail Cooper apparently sent to his bureau chief after a conversation with Karl Rove.
"It was, KR said, Wilson's wife, who apparently works at the agency on WMD [weapons of mass destruction] issues who authorised the trip," the e-mail reads.
Mr Luskin has also told Newsweek his client "never knowingly disclosed classified information" and "did not tell any reporter that Valerie Plame worked for the CIA".
Questioned by the CNN television channel last year, Mr Rove said: "I didn't know her name. I didn't leak her name."
Mr Rove is a leading strategist at the White House who was credited by President Bush as the "architect" of his 2004 election victory.
He has already testified before the investigation.
Correspondents say the case is one of the most serious legal clashes between the media and government for decades.
It has sparked concern in the US media about press freedoms and prompted calls for federal shield laws.
A number of states have legislation to protect reporters from having to identify their confidential sources.