"The fact that an official who had an important role in the Trump campaign alongside (Paul) Manafort was dealing with an individual who he knew was tied to Russian intelligence is a big deal, as is the fact that Mueller made a decision to put that card face up on the table at this time", said a person familiar with Mueller's investigation who spoke on the condition of anonymity.

Gates has pleaded guilty to conspiracy and lying to the Federal Bureau of Investigation on charges related to consulting work with pro-Russian political figures in Ukraine before joining the Trump campaign and is cooperating with Mueller's investigators.

The Russian government has denied meddling in the 2016 election, and Trump has denied any collusion by his campaign.

New documents in the Russia probe show that former Trump campaign manager, Paul Manafort, and his aide, Rick Gates, had contact with a business associate with known Russian intelligence ties, the Guardian reports. An attorney for Gates did not immediately respond to a request for comment. Van der Zwaan, who is now cooperating with the special counsel, is alleged to have withheld and destroyed documents relating to his communications with Manafort, Gates, and Person A, as well as lying to the special counsel about them.

Manafort has pleaded not guilty to charges in two indictments filed by Mueller's office.

His mentor, Manafort, has pleaded not guilty to charges of conspiracy, money laundering, and tax and bank fraud linked to his lobbying efforts for Ukraine that predate Trump's run for the US presidency.

Despite van der Zwaan's efforts, the special counsel managed to uncover that the Dutch lawyer had recorded conversations he had with Gates and Person A. In these conversations, Person A suggested that the $5 million paid to van der Zwaan's law firm through Gates and Manafort were "only a part of the iceberg" and that there were "additional payments" to come. The presidential election was in November 2016.

Four people have so far pleaded guilty as a result of the investigation led by Special Counsel Robert Mueller, and one of them Gates' associate, Alex van der Zwaan, is due to be sentenced. He also revealed that before getting involved with Manafort and Gates in 2005, he worked for the International Republican Institute, a US-funded "democracy promotion" outfit whose board of directors is now chaired by Senator John McCain (R-Arizona).

Kilimnik, whose whereabouts are now unknown, has denied having ties to Russian intelligence and Manafort has said he knew of no such connection.

Reuters was not able to locate contact information for Kilimnik, and his whereabouts are unknown. Despite the fact that Gates and Manafort knew that this business associate had ties to the Kremlin, they still chose to form a working relationship with the associate during October of 2016, mere weeks before the election.

Media reported that Manafort offered to provide Deripaska with "private briefings" about the progress of Trump's campaign in an e-mail to Kilimnik in the midst of the campaign in July 2016. This is income Manafort is accused of not reporting to the Treasury Department.

The lawsuit claims that Kilimnik graduated from the Military Institute of the Defense Ministry in Moscow.

It said an investigation by Ukrainian prosecutors into Kilimnik's possible links to Russian spy agencies was closed late previous year without charges.