A former senior official in Donald Trump’s 2016 presidential campaign, Rick Gates, has pleaded guilty to conspiracy against the United States and lying to investigators, and is cooperating with a federal probe into Russia’s role in the election.

Gates, who was a deputy campaign manager for Trump, is being investigated by the office of Special Counsel Robert Mueller, which is probing alleged Russian meddling.

He had been facing decades in prison on much more serious charges, including bank fraud and conspiracy to launder money, but under the charges he pleaded guilty to, he faces a maximum sentence of only six years.

Prosecutors said Gates could win a reduction in his sentence based on the extent of his cooperation with Mr Mueller’s probe.

The plea increases pressure on Paul Manafort, who was Mr Trump’s campaign manager for five months in 2016, to also seek a plea deal.

However, Manafort said in a statement issued after Gates’ plea deal that he maintained his innocence.

Cooperation by Gates, and potentially by Manafort at a later stage, could provide a rich vein of information for Mueller, whose Russia probe includes looking into whether Trump’s campaign colluded with Moscow to interfere in the election.

Gates’ plea deal appeared to be motivated by his concern over legal costs and the strain on his family. The sentencing guidelines for the charges he pleaded guilty to call for a prison term of between 57 and 71 months.

None of the charges to date against Gates or Manafort have made reference to any connection with Russian meddling in the 2016 election or possible collusion.

Special counsel Robert Mueller. Photo: AP

Russia has denied the accusations of interference. Trump has said there was no collusion, and has also denied any attempt to obstruct Mr Mueller’s probe.

Mr Mueller, appointed by the Department of Justice last year, has a broad brief that allows him to look into any wrongdoing uncovered in the course of his investigation.

While it was not clear what Gates might be able to reveal to investigators, he was on Mr Trump’s campaign team when Manafort, his then-boss, attended a meeting in June 2016 at Trump Tower in New York between senior campaign aides and a Russian lawyer.

Mr Mueller, according to sources familiar with the investigation, has taken a keen interest in whether Democrats’ emails allegedly hacked by Russian intelligence and made public six days after that meeting were discussed then.

Mr Mueller is also interested in differing accounts of the Trump Tower meeting, including one that was written aboard Mr Trump’s plane with the president’s assistance, the sources said.

-AAP