Manafort's spokesman Jason Maloni said, "Mr. Manafort is innocent and nothing about this latest allegation changes our defense. We will do our talking in court."
If Manafort is jailed, it could ramp up the intense pressure he already faces to plead guilty and to begin cooperating with Mueller's ongoing probe of President Donald Trump's campaign.
Manafort, 69, is scheduled to go on trial in Washington in September. But before that, he is set to be tried on related charges in Virginia federal court starting on July 24.
Manafort is charged with a slew of crimes related to his lobbying work on behalf of Ukrainian political figures who were friendly with Russia. He is accused of secretly organizing and funding a group of European political figures to lobby in the United States for Ukraine.
He has pleaded not guilty in both cases. Manafort is the only American to have been criminally charged by Mueller who has elected to go to trial.
Several others, including former Trump national security adviser Michael Flynn and Trump campaign adviser George Papadopoulos, pleaded guilty after being charged by Mueller and agreed to cooperate with the special counsel's investigation.
Manafort's business partner Rick Gates pleaded guilty in February to conspiracy and to lying to the FBI. Gates likewise is now cooperating with Mueller's team.
Since first being charged in October in the Washington case, Manafort has been under home confinement. He occasionally has been allowed to travel, albeit with certain conditions, including that he does not commit any crime. Last month, he was permitted to leave his residence to attend the baptism of his grandson.
In his court filing Monday, Mueller accused Manafort of using an encrypted messaging platform to contact potential witnesses in the case, and influence their testimony.
Mueller's prosecutors previously had accused Manafort of seeking to influence public opinion regarding his case by ghostwriting an op-ed with a colleague who prosecutors alleged was connected to Russian intelligence services.
Mueller was appointed in 2017 to investigate Russian interference in the 2016 presidential election, as well as possible cooperation by Trump campaign officials with that effort.
President Donald Trump has repeatedly called Mueller's probe a "witch hunt" and has said there was no collusion with Russia by either himself or by campaign officials.
The charges against Manafort do not include claims that he colluded with Russia during the election.
Additional reporting by Kevin Breuninger
