Paul Manafort, President Donald Trump's former campaign chairman, has been accused of attempting to tamper with potential witnesses.

The special counsel Robert Mueller asked the judge overseeing Manafort's case to revoke or revise an order releasing Manafort ahead of his trial.

Manafort is accused of attempting to communicate via phone and encrypted messaging apps with two people associated with a pro-Ukraine group involved in Manafort's lobbying activities.

The FBI says the communications were part of "an effort to influence their testimony and to otherwise conceal evidence."

If Mueller can prove to a judge that Manafort did attempt to tamper with witness testimony, it could deal a serious blow to his defense, legal experts said.

The special counsel Robert Mueller submitted a new court filing on Monday asking a judge to revoke or revise an order releasing Paul Manafort ahead of his trial.

As reasoning, the filing accused President Donald Trump's former campaign chairman of attempting "to tamper with potential witnesses" while out on bail and thus of violating the conditions of his release.

In a declaration filed with Mueller's court document, the FBI agent Brock Domin said Manafort attempted to communicate via phone and encrypted messaging with two people associated with the Hapsburg Group — a group of former senior European politicians who work to advance Ukraine's interests. They were named in the court filing as Person D1 and Person D2.

In February, Manafort was accused, among other things, of secretly paying the Hapsburg Group more than 2 million euros, through four different offshore accounts, to lobby on behalf of the Ukrainian government in 2012 and 2013, when it was controlled by the pro-Russian strongman Viktor Yanukovych.

In addition to Manafort, Mueller's office also said another person, denoted in Monday's court filing as "Person A," reached out to witnesses associated with the Hapsburg Group.

The identity of Person A is unclear.

But previous court documents indicate that the person is an associate of Manafort's, and the description bears similarities to Konstantin Kilimnik, a Russian-Ukrainian operative who has long been suspected to have ties to Russian intelligence.

According to Domin, at least one witness from the group reported Manafort's outreach and said he was attempting to influence their testimony about his lobbying activities.

The FBI agent added that the bureau had obtained documents and statements from the two people Manafort is accused of reaching out to as well as phone records and documents taken from Manafort's iCloud account that apparently show Manafort's attempts.

"Mr. Manafort is innocent and nothing about this latest allegation changes our defense," Manafort's spokesperson said in a statement. "We will do our talking in court."

Witness tampering is considered an offense against the administration of justice, according to legal experts.

"It interferes with the proper functioning of the trial," and for that reason, judges tend to view it as a serious infraction, said Jens David Ohlin, a vice dean at Cornell Law School who is an expert in criminal law. "I'd expect the judge to come down hard on Manafort and revoke bail — assuming that Mueller has solid evidence of improper contact between Manafort and the witness."

Ohlin added: "What was he thinking?"

Mueller is investigating Russian interference in the 2016 US election and whether members of the Trump campaign colluded with Moscow to help tilt the race in Trump's favor. Manafort has pleaded not guilty to dozens of charges against him including tax and bank fraud, money laundering, conspiracy against the US, failure to register as a foreign agent, and making false statements to investigators.

Manafort was released to home confinement after his arraignment in October.

His lawyers have put up an aggressive defense against Mueller since Manafort was first indicted, arguing that Mueller did not have the authority to charge Manafort with crimes unrelated to Russian collusion and that Mueller's mandate was overly broad.

But a federal judge in Washington, DC, dealt Manafort a blow last month when she ruled that Mueller was operating within the scope of his mandate when he charged the former Trump campaign chairman.

Monday's filing adds another weapon in the special counsel's arsenal.

For one, legal experts pointed out that if a witness-tampering charge is used to further restrict Manafort's conditions of release, it increases pressure on him to cooperate with the special counsel.

"Manafort is not a guy who will do well in cell block D," said Jeffrey Cramer, a former longtime federal prosecutor in Chicago who spent 12 years at the Justice Department.

Prosecutors could also introduce the charge at Manafort's trial as evidence of a guilty mind.

Moreover, the charge could also affect "the feel of the case" with respect to the judge, according to Alex Whiting, a former assistant US attorney who is now a professor at Harvard Law School. "If the judge thinks Manafort has been attempting to interfere with witnesses," the judge "will be much less likely to give the defense team the benefit of the doubt on close calls going forward," Whiting said.

Trump, meanwhile, has sought to distance himself and the campaign from Manafort since the former campaign chairman was first charged last year. Trump has denied the campaign colluded with Russia and has called Mueller's investigation a politically motivated "witch hunt."