US President Donald Trump pardoned a former White House aide convicted of lying to the FBI in connection with the leak of a CIA operative's identity -- a move seen as a message to witnesses in the current Russia probe.

"I don't know Mr. Libby," Trump said of vice president Dick Cheney's former chief of staff Lewis "Scooter" Libby.

"But for years I have heard that he has been treated unfairly," Trump added. "Hopefully, this full pardon will help rectify a very sad portion of his life."

Libby was handed a 30-month prison sentence for perjury, obstruction of justice and making false statements to investigators in 2007.

His sentence was commuted by former president George W Bush, who declined however to issue a pardon -- sparking a rift between him and Cheney.

The affair dates back to July 2003 when the name of CIA agent Valerie Plame, the wife of a diplomat who had accused the Bush administration of exaggerating the threat posed by Iraq, was leaked to the press.

Libby was not accused of blowing Plame's cover, but of lying during the subsequent inquiry.

Democrats were quick to criticize the decision -- and its possible repercussions on the probe led by Special Counsel Robert Mueller into Russian interference in the 2016 election and allegations of collusion with the Trump campaign, as well as possible obstruction of justice.

"President Trump's pardon of Scooter Libby makes clear his contempt for the rule of law," said leading congresswoman Nancy Pelosi.

"This pardon sends a troubling signal to the president's allies that obstructing justice will be rewarded," she added.

"The suggestion that those who lie under oath may be rewarded with pardons poses a threat to the integrity of the Special Counsel investigation, and to our democracy. Neither the president nor his allies are above the law."

Former vice presidential candidate Tim Kaine said it "seems like he's sending a message to current WH staff worried about investigation of Trump obstruction: "Scratch my back and I'll scratch yours."

Trump has used his power to pardon sparingly -- last year, he pardoned Arizona ex-sheriff Joe Arpaio, who was convicted of violating a court order to halt traffic patrols targeting suspected unauthorized immigrants.

The White House said that Libby had "rendered more than a decade of honorable service to the Nation as a public servant at the Department of State, the Department of Defense, and the White House."

"His record since his conviction is similarly unblemished, and he continues to be held in high regard by his colleagues and peers," White House press secretary Sarah Sanders said.

Bush critics claim Libby was part of a White House effort to punish Plame's husband, former US ambassador Joseph Wilson, whom the CIA sent to Niger in February 2002 to investigate claims that then Iraqi leader Saddam Hussein tried to buy uranium for nuclear bombs.

The White House pointed out that a "key witnesses against Mr. Libby recanted her testimony" and "the District of Columbia Court of Appeals unanimously reinstated Mr. Libby to the bar."