A former foreign policy adviser to the Trump campaign, George Papadopoulos, has secretly pleaded guilty as part of Special Prosecutor Robert Mueller's probe into Russian interference in the 2016 US presidential election.

Papadopoulos was arrested in July and pleaded guilty in October to lying to the FBI about his contact with a Russian professor with ties to Kremlin officials, prosecutors said on Tuesday. The case was only made public on Tuesday.

The case is not directly related to the explosive charging of campaign officials Paul Manafort and Rick Gates on Monday. However, both cases arose out of the investigation launched in May by Mueller to investigate whether anyone close to Trump participated in a Russian government effort to influence last year's presidential election.

Papadopoulos, who had suggested that Donald Trump meet with top Russian leaders during the campaign, is now cooperating with Mueller's probe, and his indictment lists an unidentified campaign supervisor who also was aware of contacts with the Russians.