Former U.S. national security adviser Michael Flynn pleaded guilty on Friday to lying to the FBI about contacts with Russia's ambassador, and prosecutors said he consulted with a senior official in Donald Trump's presidential transition team before speaking to the envoy.

In an appearance at a courtroom in downtown Washington, Flynn became the first member of Trump's administration to plead guilty to a crime uncovered by the special counsel investigation into Russia's alleged attempts to influence the 2016 U.S. presidential election.

Flynn, a former Army general and member of Trump's campaign team, admitted as part of a plea deal that he gave false statements to the Federal Bureau of Investigation about his contacts with Ambassador Sergei Kislyak before Trump took office.

ABC News said Flynn, facing up to five years in jail, was prepared to testify that before taking office Trump had directed him to make contact with Russians.

Reuters could not immediately verify the ABC News report on Flynn testifying, which could put the Republican president in an uncomfortable spot after he has denied any collusion between his campaign team and Moscow.

Stocks and the dollar fell sharply after the ABC report, with the benchmark S&P 500 index <.SPX> last down 1.2 percent, the dollar <.DXY> last down 0.4 percent and bond yields falling with the 10-year U.S. Treasury note US10YT=RR yielding 2.32 percent.

Flynn's 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.

The White House said Flynn's guilty plea on Friday 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.

FLYNN AND TRUMP

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

If Flynn testifies that before taking office Trump directed him to contact Russian officials, that might not necessarily amount to a crime.

If it were proven that Trump directed Flynn to lie about his contacts to the FBI, that would be a crime but legal experts disagree over whether a sitting president can be indicted.

Many say the only clear punishment for a president who has committed criminal acts is impeachment by Congress. The Constitution provides that impeachment, which requires a simple majority in the House and a two-thirds vote in the Senate, is warranted for “high crimes and misdemeanors,” which Congress is free to define as it sees fit.

Moscow has denied what U.S. intelligence agencies say was meddling in the election campaign to try to sway the vote in Trump's favour. Trump has called Mueller's probe a witch hunt.

Prosecutors said that Flynn and Kislyak last December discussed a diplomatic dispute over economic sanctions that Washington had imposed on Moscow, and an upcoming vote in the U.N. Security Council regarded as damaging to Israel.

Flynn admitted falsely telling FBI officials in January that he did not ask the ambassador to refrain from escalating a diplomatic dispute over U.S. sanctions on Moscow.

The Obama administration, which was still in office at the time, had imposed the sanctions on Moscow for allegedly interfering in the election.

Flynn also lied about asking the envoy to help delay a vote in the U. N. Security Council that was seen as damaging to Israel.

Flynn consulted with a senior member of Trump's presidential transition team about “what, if anything, to communicate to the Russian ambassador about the U.S. sanctions,” prosecutors said in a court document.

Flynn called a senior official of Trump’s transition team who was with other members of the team at the Mar-a-Lago resort in Florida, the prosecutors said,

"Flynn called the Russian ambassador and requested that Russia not escalate the situation and only respond to the U.S. sanctions in a reciprocal manner,” the document said. It did not name the senior official in the Trump team.

GUILTY PLEA

Flynn appeared calm as he pleaded guilty in a packed federal courtroom in Washington. His wife, Lori, sat in the first row of benches in the courtroom.

Occasionally as Flynn stood to answer routine questions, his attorney Robert Kelner put his hand on Flynn’s back.

“Guilty, your honour,” Flynn said, when asked by U.S. District Judge Rudolph Contreras how he planned to plea.

Flynn was known for vitriolic campaign appearances in 2016, notably leading Trump supporters' chants of "Lock Her Up," in reference to Trump's Democratic rival, Hillary Clinton, and accusations she misused email while she was secretary of state.

He is the second former senior aide to Trump to be charged in the probe.

Paul Manafort, who ran Trump's presidential campaign for several months last year, was charged in October with conspiring to launder money, conspiracy against the United States and failing to register as a foreign agent of Ukraine’s former pro-Russian government.

Manafort, who did not join Trump's administration, and a business associate who was charged with him, pleaded not guilty.