WASHINGTON – President Trump on Tuesday called George Papadopoulos – his campaign foreign policy adviser who pleaded guilty to lying about his Russian contacts to FBI agents – a “young, low level volunteer” and a “liar." 

As part of Robert Mueller's investigation into Russia's interference in the 2016 presidential election, Papadopoulos pleaded guilty to making false statements and “material omissions” to the FBI about several communications he had with allies of the Russian government, including a professor who offered him "dirt" on Trump’s election opponent, Hillary Clinton.

Additional charges were filed Monday against two major players in Trump’s campaign: former campaign chairman Paul Manafort and his business associate Rick Gates, who also worked on Trump's campaign.

More: Mueller's bombshell: Special counsel charges Manafort, Gates and reveals aide's Russia contacts

More: Ex-Trump campaign adviser George Papadopoulos pleads guilty to lying to FBI about Russian contacts

In an early morning tweet, Trump blasted the idea that there was any coordination with Russia, saying "the Fake News is working overtime." 

He distanced himself from Papadopoulos, 30. "Few people knew the young, low level volunteer named George, who has already proven to be a liar. Check the DEMS!"

The series of charges unsealed early Monday morning is a significant step in the federal investigation of Russian attempts to influence the 2016 presidential election, which Trump has previously dismissed as a “hoax” and a “witch hunt.” 

Yet the president pointed to Tony Podesta, a Democratic lobbyist and brother to former Clinton campaign chairman John Podesta, who has become a subject of Mueller's investigation as "the biggest story yesterday."

According to reports, Podesta failed to disclose lobbying activity for a Ukrainian nonprofit that has been tied to Manafort. In light of the probe, Podesta stepped down from his firm, the Podesta Group, on Monday.

The president’s attempts to minimize Papadopolous’s role in his campaign mirrors the approach by his spokeswoman on Monday, Sarah Sanders, who also called Papadopolous a volunteer and said Papadopoulos's role in the campaign was "extremely limited." 

"No activity was ever done in an official capacity on behalf of the campaign," Sanders said Monday. 

More: White House: Confident, but concerned about the unknown after legal action

While Mueller's filings did not allege there was collusion between the campaign and Russia, Papadopoulos’s attempts to contact Russians did take place during the campaign. Legal documents filed by Mueller show he was reporting to – and being encouraged by – top-level aides in the campaign. 

The former aide acknowledged in a court filing that he misled FBI agents about his conversations with a professor who told him the Russian government had obtained "thousands of emails" with dirt on Clinton. The exchange came amid widespread publicity about Clinton's use of a private email server, and months before the anti-secrecy group WikiLeaks released internal Democratic National Committee emails that cast an embarrassing light on her campaign. U.S. intelligence agencies have concluded that the DNC's emails were hacked at the behest of the Russian government. 

Papadopoulos also tried repeatedly to set up meetings between the Russian government and Trump campaign officials, with the help of the professor and other well-connected Russian contacts, the court filing shows. 

Papadopolous pleaded guilty to the charges on Oct. 5, but it was kept secret until Monday because he was working with the government as a "proactive cooperator” in the Russia probe, according to the government's motion to seal his arrest and plea records.

– Contributing: Deirdre Shesgreen, Brad Heath