US President Donald Trump’s former campaign chairman Paul Manafort has agreed to co-operate with the special counsel’s Trump-Russia investigation.

Manafort has pleaded guilty to federal crimes and avoided a second trial that could have exposed him to more time in prison.

The deal gives special counsel Robert Mueller a key co-operator who steered the Trump election effort for a pivotal stretch of the 2016 presidential campaign.

The result also ensures the investigation will extend far beyond the November congressional elections despite entreaties from the president’s lawyers that Mr Mueller bring it to a close.

It is unclear what information Manafort is prepared to offer investigators about the president or that could aid Mr Mueller’s investigation into Russian interference in the 2016 election.

But his involvement in key episodes under scrutiny, and his leadership of the campaign at a time when prosecutors say Russian intelligence was working to sway the election, may make him an especially valuable witness.

The agreement makes Manafort the latest associate of Mr Trump, a president known to place a premium on loyalty among subordinates, to admit guilt and work with investigators in hopes of leniency.

Manafort had long resisted the idea of co-operating even as prosecutors stacked additional charges against him in Washington and Virginia.

Mr Trump had saluted that stance, publicly praising him and suggesting Manafort had been treated worse than gangster Al Capone. Mr Trump’s lawyer, Rudy Giuliani, had suggested a pardon might be a possibility after the investigation was concluded.

Then came Friday’s development when Manafort agreed to provide any information asked of him, testify whenever asked and even work undercover if necessary.

Mr Mueller has already secured cooperation from a former national security adviser who lied to the FBI about discussing sanctions with a Russian ambassador, a campaign aide who broached the idea of a meeting with Russian President Vladimir Putin; and another aide who was indicted alongside Manafort but ultimately turned on him. Mr Trump’s former personal lawyer has separately pleaded guilty in New York.

Donald Trump
Donald Trump’s lawyer Rudy Giuliani had suggested the US President might pardon Manafort (AP Photo/Susan Walsh, File)

Friday’s deal, to charges in Washington tied to Ukrainian political consulting work but unrelated to the campaign, was struck just days before Manafort was to stand trial for a second time.

He 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. The two conspiracy counts he admitted to on Friday carry up to five years, though Manafort’s sentence will ultimately depend on his co-operation.

“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 lawyer Kevin Downing said outside court.

The agreement does not specify what, if anything, prosecutors hope to receive about Mr Trump, but Manafort could be well-positioned to provide key insight for investigators working to establish whether the campaign co-ordinated with Russia.

He was among the participants, for instance, in a June 2016 Trump Tower meeting with Russians and Mr Trump’s oldest son and son-in-law that was arranged for the campaign to receive derogatory information about Democrat Hillary Clinton.

He was also a close business associate of a man who US intelligence believes has ties to Russian intelligence. And while he was working on the campaign, emails show Manafort discussed providing private briefings for a wealthy Russian businessman close to Mr Putin.

White House press secretary Sarah Sanders insisted the Manafort case was unrelated to Mr Trump, and Mr Giuliani said: “Once again an investigation has concluded with a plea having nothing to do with President Trump or the Trump campaign.”