Donald Trump's former campaign chairman Paul Manafort will cooperate with the federal investigation into Russian meddling in the 2016 election, after months of refusing to do so.
Manafort took a plea deal and agreed to cooperate with Special Counsel Robert Mueller's inquiry into Russian interference and possible coordination between Mr Trump campaign members and Moscow. He did so in return for reduced charges.
It was not immediately clear what information Manafort, a long-time Republican political consultant who ran the campaign as it took off in mid-2016, could offer prosecutors about Mr Trump.
But Friday's announcement was a political blow to the presidency ahead of 6 November mid-term elections that could determine whether or not Republicans maintain control of Congress.
Manafort, 69, pleaded guilty in a federal court in Washington on Friday to counts of conspiracy against the United States and conspiracy to obstruct justice, becoming the most prominent former of Mr Trump's campaign officials to plead guilty in Mr Mueller's investigation.
He had other charges dropped but could still face 10 years in prison on the two charges in Washington. In August, Manafort was convicted at trial in Alexandria, Virginia, on charges that pre-dated his stint on Mr Trump's campaign and involved his work with pro-Russian politicians in Ukraine.
In a statement, the White House distanced Mr Trump from the man who helped get him elected in November 2016 against the odds in a bitterly contested campaign in which he defeated Democrat Hillary Clinton.
"This had absolutely nothing to do with the president or his victorious 2016 presidential campaign," White House spokeswoman Sarah Sanders said.
The probe, which Mr Trump often describes as "witch hunt," has cast a shadow over the presidency.
Manafort had refused to cooperate with Mr Mueller even as the Virginia jury convicted him on bank and tax fraud charges.
Prosecutors had accused him of hiding from US tax authorities $16 million ($NZ24.4m) he earned as a political consultant in Ukraine to fund an opulent lifestyle and then lying to banks to secure $20m ($NZ30.5m) in loans.
Trump last month praised his former campaign aide for not entering into an agreement with prosecutors, as the president's former personal lawyer Michael Cohen had.
On 22 August, Trump wrote on Twitter: "Unlike Michael Cohen, he refused to 'break' - make up stories in order to get a 'deal. Such respect for a brave man!"
Jury selection was due to begin on Monday in a second Manafort trial on charges including conspiring to launder money, conspiring to defraud the US, failing to register as a foreign agent and witness tampering.
Instead Manafort entered the plea.
Manafort made millions of dollars working in Ukraine before taking an unpaid position with Trump's campaign for five months.
He led the campaign when Trump was selected as the Republican presidential nominee at the party convention.
Moscow has denied interfering in the 2016 election and Mr Trump has said there was no collusion.
Former federal prosecutor Harry Sandick said it was too soon to say how much Manafort's cooperation could affect the investigation into possible collusion between Trump's campaign and the Russians.