Prosecutors with Special Counsel Robert Mueller said today that Paul Manafort breached his plea agreement by lying repeatedly as they questioned him in the investigation into Russian interference in the 2016 election.

Manafort denies doing so, and both sides agree that sentencing should be set immediately.

The apparent collapse of Manafort's cooperation agreement is the latest stunning turnaround in his case, exposing the longtime Republican consultant to more than a decade behind bars after pleading guilty in September on charges of cheating the Internal Revenue Service, violating foreign lobbying laws and attempting to obstruct justice.

The court filing indicated Mueller's team also had suffered a potential setback, after gaining access to a witness with potential knowledge of several key events relevant to the probe during his tenure with Donald Trump's campaign from March to August 2016, including a Trump Tower meeting attended by a Russian lawyer and the Republican National Convention.

"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, which constitute breaches of the agreement," prosecutors wrote.

"The government will file a detailed sentencing submission to the Probation Department and the Court in advance of sentencing that sets forth the nature of the defendant's crimes and lies."

Prosecutors did not elaborate on areas where they contend Manafort lied.

Manafort disputes that characterisation; his lawyers write in the joint filing that he "has provided information to the government in an effort to live up to his cooperation obligations".

Manafort pleaded guilty on September 14 days before trial in Washington, DC, to two charges — conspiring to defraud the United States and conspiring to obstruct justice — admitting to years of financial crimes related to his undisclosed lobbying work for a pro-Russian political party and politician in Ukraine.

Under an agreement with prosecutors, Manafort faces a maximum prison sentence of about 10 years, not counting a sentence for his August conviction in Virginia for bank and tax fraud. He was ordered to forfeit an estimated US$15 million he hid from the Internal Revenue Service, but permitted to keep some property held with family members.

Before the plea, Manafort's defenders had long insisted that he would not cooperate with Mueller.

But in plea papers, Manafort agreed to cooperate "fully and truthfully" with the investigation conducted by the office of special counsel, including participating in interviews and debriefings, producing any documents in his control, testifying, and agreeing to delay sentencing until a time set by the government.

In return for his cooperation, Manafort hoped to have prosecutors recommend leniency, possibly slicing years off his term of incarceration.

The filing came after Manafort talked in detail to prosecutors before his plea, and despite numerous visits by him and his lawyers since to prosecutors' offices.

Kevin Downing, a lawyer for Manafort, said at the time of Manafort's plea that it included a full cooperation agreement. "He wanted to make sure his family remained safe and live a good life," Downing said outside the courthouse doors. "He has accepted responsibility."