Disgraced cyclist Lance Armstrong could be under US criminal investigation, despite a claim that is not the case.

An American media report has contradicted a claim by California attorney Andre Birotte that his department had no plans to press charges against Armstrong.

America's ABC News has quoted an anonymous source as saying: "Birotte does not speak for the federal government as a whole.

"Agents are actively investigating Armstrong for obstruction, witness tampering and intimidation."

The claim was made only hours after Birotte spoke about Armstrong.

He led a federal investigation into Armstrong that ended suddenly a year ago without any charges being laid.

Birotte did not definitively rule out action, but said Armstrong's public admission had not yet changed the decision not to prosecute.

"We made a decision on that case, I believe, a little over a year ago," he said, when asked about the status of the federal inquiry into long-standing claims that Armstrong had run a doping program and had lied to federal agents.

"Obviously we've been well aware of the statements that have been made by Mr Armstrong and other media reports," he said, referring to Armstrong's bombshell confession on chat show legend Oprah Winfrey's broadcast last month.

"That has not changed my view at this time. Obviously we'll consider - we'll continue to look at the situation, but that hasn't changed our view as I stand here today," Birotte said at a news conference in Washington on Tuesday.

The 41-year-old Texan rider was stripped of his record seven Tour de France titles last year after the US anti-doping agency gathered compelling testimony that he had been the ringleader of a large-scale doping conspiracy.

He had long angrily protested his innocence, including in questioning by US federal agents investigating the same allegations, but the mask fell away last month when he confessed his guilt to Oprah Winfrey in detail.

The confession threw up a number of legal questions, including whether the federal probe might be reopened, if he might be prosecuted for perjury and whether sponsors might sue to recover payments and prize money.

Armstrong also faces other legal battles after being stripped of his record seven Tour titles.

Dallas insurance company SCA Promotions has already demanded the return of $US12 million ($A11.6 million) in bonuses it paid to Armstrong for multiple Tour victories, and SCA attorney Jeff Dorough told AFP that the firm expected to file a lawsuit against Armstrong as early as Wednesday.

"We are still pursuing the suit, and we expect to file tomorrow," Dorough said.

SCA withheld a $US5 million ($A4.8 million) bonus due after Armstrong's sixth Tour de France win in 2004 because of doping allegations circulating in Europe, and Armstrong took them to court.

He won the case because SCA's original contract had no stipulations about doping, and Armstrong attorney Tim Herman told USA Today that the shamed cyclist doesn't intend to pay back any of the money.

"My only point is no athlete ever, to my understanding, has gone back and paid back his compensation," Herman told the newspaper in an article published on Tuesday.