Donald Trump's former campaign chairman Paul Manafort has breached his plea deal by lying to federal investigators, according to a court filing, in a potential setback to the special counsel's probe into Russia's meddling in the 2016 US presidential election.
Manafort said in the same filing that he disagreed with Special Counsel Robert Mueller's assertion that he had lied, but both sides agreed the impasse meant that the court should move ahead and sentence him for his crimes.
Without a pardon, Manafort could spend the rest of his life in prison, experts said.
The surprise development came at a critical time for Mueller, who is expected to finalise a report in the coming months on the findings of his 18-month probe into Russia's election meddling and possible collusion between Moscow and the Trump campaign.
Paul Manafort, former campaign chairman for US President Donald Trump. Source: AAP
While not a fatal blow, the failure to secure Manafort's cooperation has damaged the credibility of an individual with deep ties to Russia and who prosecutors had hoped would prove to be a valuable witness, a former federal prosecutor said.
"It's bad for the overall Mueller investigation," said Patrick Cotter, a criminal defence lawyer in Chicago and former assistant US lawyer in New York. "He's got one less witness today."
Manafort, a longtime Republican political consultant who made tens of millions of dollars working for pro-Kremlin politicians in Ukraine, ran the Trump campaign as it took off in mid-2016. He attended a meeting at Trump Tower in June 2016 with a group of Russians offering "dirt" on Democratic candidate Hillary Clinton, who lost in an upset to Trump in the presidential vote that November.
Manafort had started cooperating with Mueller in September after pleading guilty in a federal court in Washington to conspiracy against the United States - a charge that included a range of conduct from money laundering to unregistered lobbying - and attempting to tamper with witnesses.
Manafort, a longtime Republican political consultant ran the Trump campaign. Source: AAP
Mueller said in the filing that after signing the plea agreement: "Manafort committed federal crimes by lying to the Federal Bureau of Investigation and the Special Counsel's Office on a variety of subject matters."
Prosecutors did not provide details of the alleged lies but said they would do so prior to sentencing.
Manafort's lawyers said in the same filing submitted to US District Judge Amy Berman Jackson in Washington that Manafort had met with the government on several occasions and made "an effort to live up to his cooperation obligations".
Russia denies US allegations it hacked Democratic Party emails and ran a disinformation campaign, largely on social media. Trump denies any campaign collusion and calls the investigation a political witch hunt.
