A federal judge has sentenced US President Donald Trump's former deputy campaign chairman to 45 days in jail, saying that while she was moved by his extensive cooperation with prosecutors, she still felt some punishment was in order.

Key points: Rick Gates initially pleaded not guilty but later agreed to cooperate with prosecutors

Rick Gates initially pleaded not guilty but later agreed to cooperate with prosecutors He may be able to serve his sentence on weekends

He may be able to serve his sentence on weekends Gates testified against Paul Manafort, who was convicted of fraud in August

Rick Gates, who was also sentenced to 36 months of probation, held senior posts on the President's campaign team and inauguration committee.

He pleaded guilty to charges related to tax fraud, concealing foreign bank accounts and violating the Foreign Agents Registration Act regarding work done for Ukraine's former pro-Russia government.

He was one of several former Trump aides and advisers to be charged in the Mueller investigation that documented Russian interference in the 2016 US election.

Gates, 47, initially pleaded not guilty but later agreed to cooperate with prosecutors.

US District Judge Amy Berman Jackson said she had struggled with how to sentence Gates, who met with investigators more than 50 times and helped the Government secure convictions of Republican operatives Paul Manafort and Roger Stone after he pleaded guilty in February 2018 to lying to the FBI and the special counsel's office and conspiring against the United States.

Judge Jackson said that every day she sees people who commit crimes "of a much more limited scope" than Gates's offences who are motivated by drug addictions and poverty, and who like Gates, become government witnesses by turning on their own friends and neighbours.

"Even they often serve some time," she.

Several of Mr Trump's former allies and advisors have been charged with crimes or already sentenced to prison. ( AP: Pablo Martinez Monsivais )

Judge Jackson also said Gates will be required to pay a $US20,000 ($29,206) fine, complete community service and continue cooperating in several still-ongoing investigations.

Possible jail on weekends

The judge gave Gates leeway to work with the probation office to iron out the terms of when he will serve his 45 days in jail, saying he could do so on weekends or during times that will not interfere with his parenting responsibilities or his ongoing cooperation with law enforcement.

"I wish to express to this court that I accept complete responsibility for my actions that have led me here," Gates said.

"I greatly regret the mistakes I have made and I have worked hard to honour my commitment to make amends."

Under the sentencing guidelines calculated by the government, he could have faced up to four years in prison.

Gates's defence attorney Thomas Green had tried to persuade Judge Jackson to sentence his client solely to probation, and prosecutors did not push back on that recommendation.

Mr Green said he saw a number of investigators whom Gates had cooperated with at the courtroom, and they were not just there out of curiosity.

"I am quite certain they are here to acknowledge the genuine contributions that Mr Gates made to their continuing investigations," he told the judge.

Star witness against Paul Manafort

Paul Manafort, President Donald Trump's former campaign chairman, is serving a seven-and-a-half-year prison sentence. ( AP: Andrew Harnik )

Gates appeared as a star witness for the prosecution in three trials.

He testified against Paul Manafort, his former business partner and mentor, who was convicted in Virginia of financial fraud in August 2018 and is now serving a seven-and-a-half-year prison sentence.

Manafort served as Mr Trump's campaign chairman.

Last month, Gates testified against Roger Stone, Trump's longtime adviser who was convicted by a jury in November in Washington of lying to Congress, obstruction and witness tampering.

He will be sentenced in February.

Roger Stone, a longtime ally of Mr Trump, will be sentenced in February. ( Reuters: Kevin Lamarque )

Gates also testified against Greg Craig, who served as White House counsel under Democratic former president Barack Obama and was acquitted in Washington in September of lying to authorities about work he performed for Ukraine.

Until his decision to cooperate, Gates faced more serious charges, which included helping Manafort file false tax returns, subscribing to false tax returns about his own income and lying about the existence of overseas accounts.

Judge Jackson said she believed Gates was sincere and appeared to be a very credible Government witness who "didn't come across as some sort of bought-and-paid-for puppet" for the Government.

Nevertheless, she said, he was "hardly a minor player" in the fraud, and she rejected the notion he somehow got swept into the drama of Washington politics.

"Politics don't corrupt people. People corrupt politics," she said.

Reuters