Donald Trump's former campaign chairman has pleaded guilty to two federal crimes after cutting a deal with US prosecutors and agreeing to cooperate with the special counsel's Russia probe.

The move allows Paul Manafort to avoid a second criminal trial and ends his fight against investigators in the Russia probe.

Manafort was convicted last month of eight financial crimes in a separate trial in Virginia and faces seven to 10 years in prison in that case.

On Friday, prosecutor Andrew Weissman said in court that Manafort had struck a "cooperation agreement" and would plead guilty to charges related to his Ukrainian political consulting work.

"He wanted to make sure that his family was able to remain safe and live a good life. He's accepted responsibility. This is for conduct that dates back many years and everybody should remember that," Manafort's attorney Kevin Downing said.

The charges do not relate to Russian interference in the 2016 presidential election, which is the central issue in the special counsel's investigation into possible contacts between Trump's campaign and Russia.

Still, the move gives Mueller another successful conviction while allowing Manafort to avoid facing another costly public trial that this time focused on allegations that he acted as an unregistered foreign agent for Ukrainian interests.

Mueller also gained a key cooperator in Manafort, who participated in a June 2016 Trump Tower meeting with a Russian lawyer that Donald Trump Jr attended despite it being described as part of a Russian government effort to aid his father's campaign.

A grand jury used by Mueller has heard testimony about the meeting.

White House press secretary Sarah Sanders said the Manafort case has nothing to do with Trump.

"This had absolutely nothing to do with the President or his victorious 2016 Presidential campaign. It is totally unrelated."

"The president did nothing wrong," said Rudy Giuliani, Trump's attorney, in a statement. "Once again an investigation has concluded with a plea having nothing to do with President Trump or the Trump campaign."

Under the terms of Friday's plea deal, prosecutors dropped the bulk of the charges against Manafort, filing new paperwork that includes just two counts that resemble in many ways the original allegations made in an indictment last year.

The charges include conspiracy against the United States and conspiracy to obstruct justice.

It is unclear how the possible deal might affect Manafort's pursuit of a pardon from President Donald Trump.

The president has signalled that he is sympathetic to Manafort's cause.

Manafort has aggressively fought the charges against him and taken shots at his co-defendant, Rick Gates, who cut a deal with prosecutors earlier this year that included a cooperation agreement.