United States 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".

Key points: Donald Trump says George Papadopoulos was a low-level volunteer

Donald Trump says George Papadopoulos was a low-level volunteer Legal experts say Papadopoulos may have been working closely with FBI recently

Legal experts say Papadopoulos may have been working closely with FBI recently Kremlin says charges prove Russia didn't interfere in US election

Mr Trump sought to dismiss George Papadopoulos, who has provided key evidence in the first criminal case connecting Mr 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!" Mr Trump tweeted.

Mr Trump had called Mr Papadopoulos an "excellent guy" last March and tweeted a picture with him at a "national security meeting" in Washington.

George Papadopoulos, left, sits at a table with then-candidate Donald Trump at what was labelled a "national security meeting" in 2016. ( AP: Donald Trump Twitter )

Mr 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.

Mr Papadopoulos pleaded guilty to lying to FBI agents about the conversations and has been cooperating with investigators, the documents said.

His guilty plea and the possibility that he's working with Mr Mueller's team came as an unexpected twist in the mounting drama surrounding the criminal probe.

"There's every chance that he [Mr Papadopoulos] was possibly wearing a wire for the FBI or otherwise cooperating more recently than has been suspected," former US deputy assistant attorney-general Robert Driscoll told the ABC's The World program.

"We don't know that happened, but I think there is widespread speculation within the legal community given the timing of everything that that's a possibility.

"So I think that for folks that have been interacting with him, they're going to have second thoughts about what exactly it is they've said to him since around mid-July."

Sorry, this video has expired Former FBI agent says Mueller's first charges part of wider strategy

'It's a laughable allegation': Kremlin

White House spokeswoman Sarah Huckabee Sanders said Mr Papadopoulos' role was "extremely limited" and that "no activity was ever done in an official capacity on behalf of the campaign".

Mr Trump named Mr Papadopoulos to his foreign policy advisory council in March 2016.

During a meeting in March 2016 with The Washington Post editorial board, Mr Trump called Mr Papadopoulos an "excellent guy".

Shortly afterwards, Mr Trump tweeted a photo of his advisory council meeting, with Mr Papadopoulos among a handful of advisers at the President's table.

In his plea filing, Mr Papadopolous admitted that he told Mr Trump and other top campaign national security officials during that meeting that he had made contact with intermediaries for Russia who said they could set up a meeting between Mr Trump and Russian President Vladimir Putin.

The Kremlin responded to the claims on Tuesday, saying accusations someone with links to the Russian Foreign Ministry may have tried to set up a Putin-Trump meeting was unsubstantiated.

Sorry, this video has expired Paul Manafort leaves the Federal District Court in Washington. (Photo:AP/Alex Brandon)

"It's an absolutely laughable allegation," Kremlin spokesman Dmitry Peskov said.

The Kremlin also said charges against Mr Trump's former campaign manager Paul Manafort and another aide showed Moscow had been unfairly maligned for meddling in last year's US presidential election.

Federal investigators probing alleged Russian interference in the election filed a series of charges against Mr Manafort and Rick Gates on Monday, including conspiracy against the US and money laundering.

Mr Peskov highlighted the absence of allegations against Russia, saying that Moscow had always said it had never meddled in the US election.

"Russia does not feature in the charges that were levelled in any way," Mr Peskov said.

"Other countries and other people feature [in the charges]."

Russia's flagship TV news show on Monday evening said it was "now clear that there was nothing" to allegations about Mr Manafort being in touch with Russian officials to try to sway the election.

The US investigation was an internal matter for the United States, said Mr Peskov, but Moscow was following it with interest.

Sorry, this video has expired Sarah Huckabee Sanders distances Donald Trump from Robert Mueller's indictments

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