President Donald Trump says a former campaign aide thrust into the centre of special counsel Robert Mueller's Russia probe "has already proven to be a liar".
On Tuesday, Trump dismissed George Papadopoulos, who has provided key evidence in the first criminal case connecting Trump's team to alleged intermediaries for Russia's government.
"Few people knew the young, low level volunteer named George, who has already proven to be a liar. Check the DEMS!" the president tweeted.
Papadopoulos was approached by people claiming ties to Russia and offering "dirt" on Hillary Clinton in the form of thousands of emails, according to court documents unsealed on Monday.
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Papadopoulos pleaded guilty to lying to FBI agents about the conversations and has been cooperating with investigators, the documents said.
Papadopoulos' guilty plea and the possibility that he's working with Mueller's team came as an unexpected twist in the mounting drama surrounding the criminal probe.
A separate welter of charges Mueller announced against Trump's ex-campaign chairman Paul Manafort and his longtime aide Rick Gates do not appear directly related to their work for Trump.
Papadopoulos was arrested in July and has been interviewed repeatedly by authorities.
After entering his guilty plea he was ordered not to contact other Trump officials and prohibited from foreign travel.
In one of the unsealed files, an FBI agent working for Mueller bluntly hinted that more former Trump associates could soon be questioned.
Papadopoulos' lawyer, Thomas M. Breen, based in Chicago, declined to comment on the guilty plea.
The incident echoes elements of a June 2016 meeting involving Donald Trump Jr and other campaign officials at Trump Tower.
The president's son organised that sit down with a Russian lawyer who was offering negative information about Clinton.
The White House immediately cast Papadopoulos as a mere volunteer with little influence during last year's campaign.
