Donald Trump's former aide Steve Bannon was sentenced to four months in prison on Friday over his refusal to testify in the congressional probe of the January 6, 2021 attack on the Capitol.

One of the masterminds behind Trump's 2016 presidential campaign and victory, Bannon was found guilty in July on two counts of contempt of Congress for defying a subpoena to testify over the riot by the former president's supporters.

Bannon was fined US$6,500, and was permitted by the judge to remain free while he fights what his lawyer vowed would be a "bulletproof" appeal.

The prison sentence was less than the six months the Justice Department had requested, but more than probation that Bannon's attorneys had sought.

Bannon had argued that he declined to appear before the congressional probe upon advice from his lawyer that doing so would violate Trump's executive privilege.

But he also felt that the January 6 committee investigation was politically driven.

Federal Judge Carl Nichols however rejected the arguments, saying Trump had never asserted executive privilege in Bannon's case and that the events of January 6 needed investigating.

"The events of January 6th were undeniably serious," Nichols said before pronouncing the sentence.

"The January 6 committee thus has every reason to investigate what happened that day."

In addition, he said, Bannon did not even cooperate with the committee on issues not touched by possible executive privilege.

Bannon has "not produced a single document... and has not provided any testimony on any topic," he said. (AFP)