Former Trump national security adviser Michael Flynn leaves federal court in Washington. Flynn pleaded guilty to making false statements to the FBI, the first Trump White House official to make a guilty plea so far. Picture: Susan Walsh/AP

Washington - Former US national security adviser Michael Flynn pleaded guilty on Friday to lying to the FBI about Russia, and ABC News said he was prepared to testify that before taking office President Donald Trump had directed him to make contact with Russians.

Reuters could not immediately verify the ABC report, which cited a Flynn confidant. The news sent U.S. stocks sharply lower.

Flynn, a former top Trump campaign aide and a central figure in a federal investigation into Moscow's alleged interference in the 2016 U.S. presidential election, pleaded guilty under a plea deal on Friday to lying to the FBI.

His decision to cooperate with the investigation led by Special Counsel Robert Mueller, marked a major escalation in a probe that has dogged Trump's administration since the Republican president took office in January.

ABC News cited a confidant as saying Flynn was ready to testify that Trump directed him to make contact with Russians, initially as a way to work together to fight the Islamic State group in Syria.

It was not clear when Trump supposedly told Flynn to contact Russians and whether there would be anything illegal in requesting such contact.

Flynn pleaded guilty to making false statements about contacts he had in December with Russia's ambassador to the United States, Sergei Kislyak, during the transition period after Trump was elected and before he was sworn in. The charges carry a sentence of up to five years in prison.

The White House said Flynn's guilty plea implicated him alone.

"Nothing about the guilty plea or the charge implicates anyone other than Mr. Flynn," Ty Cobb, a White House attorney, said in a statement on Friday.

Flynn was forced out of his White House post in February for misleading Vice President Mike Pence about his conversations with the ambassador.

"The false statements involved mirror the false statements to White House officials which resulted in his resignation in February of this year," Cobb said, adding that the plea "clears the way for a prompt and reasonable conclusion" of Mueller's probe.

Moscow has denied a conclusion by U.S. intelligence agencies that it meddled in the election campaign to try to sway the vote in Trump's favor. Trump has denied any collusion by his campaign and has called Mueller's probe a witch hunt.

Flynn said on Friday his decision to plead guilty and to cooperate with the investigation was "made in the best interests of my family and of our country."

In a statement issued by the law firm representing him, Flynn also said it was "painful to endure" the "false accusations of 'treason' and other outrageous acts" over the past several months but that he recognized "that the actions I acknowledged in court today were wrong."