Former president Donald Trump advisor Steve Bannon expressed defiance Monday after surrendering to the FBI to face charges of contempt of Congress after refusing to testify on the January 6 Capitol assault.

He accused President Joe Biden of having "ordered" the Justice Department to prosecute him.

"I'm never going to back down," he told reporters after appearing before a judge to hear the charges.

"We're going on the offense on this. And stand by," he said, repeating the phrase Trump used during the election last year to encourage supporters of a far-right militia group.

On Friday, acting on a referral from Congress, a federal grand jury indicted Bannon for snubbing a subpoena to testify and supply documents to the House Select Committee investigating the violent January 6 attack on the US Congress by Trump supporters.

Investigators believe Bannon and other advisors to Trump could have information on links between the White House and the mob that invaded the Capitol, on the day it was due to certify Joe Biden as winner of the November 2020 presidential election.

But, although he was not a White House employee or official Trump advisor, Bannon's attorneys said he was protected by presidential privilege and did not have to cooperate with the committee.

January 6 'war room'

Bannon, 67, was indicted by a grand jury with two misdemeanour counts of contempt, each one carrying a penalty of one month to one year in jail, and a fine of up to $100,000.

The indictment was a significant victory for the House Select Committee, which is battling efforts by Trump to use his privilege as a former president to block the committee from obtaining testimony and documents necessary for the probe.

"Steve Bannon's indictment should send a clear message to anyone who thinks they can ignore the Select Committee or try to stonewall our investigation: no one is above the law," said committee chair Bennie Thompson and vice chair Liz Cheney in a statement on Friday.

Bannon was formally advised of the charges in Washington federal court and released. He will appear for an initial virtual hearing on Thursday.

Bannon was among dozens of people who have been called to testify on the violent attack aimed at shutting down Congress over Trump's baseless claims that Biden had won the election due to voter fraud.

The attack, which left five people dead, succeeded in delaying the joint House-Senate election certification session for several hours.

The committee is probing whether people close to and advising Trump encouraged or possibly helped steer Trump's supporters into attacking the Capitol.

The investigators said Bannon was present in a downtown Washington hotel where Trump associates operated what one described as a "war room" on January 5 and 6, allegedly communicating with both protestors and the White House.

Fight over privilege

The committee says it has already interviewed or obtained documents from more than 150 people, and has demanded testimony from a number of top Trump aides.

A key witness, Trump's former chief of staff Mark Meadows, could also face charges of contempt after refusing to be interviewed by the committee on Friday.

Bannon's attorney David Schoen criticized the Justice Department for treating his misdemeanour charges as a "capital case."

"We going to challenge this affirmatively," he said.

Bannon's and Meadows' cases could hinge on an ongoing court fight between the committee and Trump over executive privilege.

Trump is fighting the committee's request for documents from his administration now in the National Archives.

After Biden, as serving president, waived privilege over the documents, the federal court in Washington rejected Trump's challenge.

Trump has since appealed, and the case, which could go to the Supreme Court, is now focused on never-before-tested clashing privilege stances by a serving and former president.