Comey says Republicans "have to have the courage to stand up and speak the truth, not be cowed by mean tweets or fear of their base".

Comey, who was fired by Trump in May 2017 while he was leading an investigation into Russian interference in the 2016 US election and possible Trump campaign collusion, made his remarks after his second appearance this month before two House of Representatives committees.

Comey said lawmakers had again asked about former secretary of state Hillary Clinton's emails and the Steele dossier - two favourite subjects of Republicans who insist there was bias in the department.

The dossier was Democratic-funded opposition research on Trump's ties to Russia compiled by former British spy Christopher Steele.

"This while the president of the United States is lying about the FBI, attacking the FBI and attacking the rule of law in this country. How does that make any sense at all?" Comey told reporters after spending more than five hours being interviewed behind closed doors by the House judiciary and oversight committees.

Trump has repeatedly gone after the FBI for bias as his campaign has been under investigation. He has called special counsel Robert Mueller's Russia investigation a "hoax".

On Sunday, he called his former personal lawyer, Michael Cohen, a "rat" because he has co-operated with prosecutors.

White House Press Secretary Sarah Sanders responded on Monday night on Twitter, writing: "Republicans should stand up to Comey and his tremendous corruption. ... The President did the country a service by firing him and exposing him for the shameless fraud he is."

The House judiciary and oversight and government reform committees are wrapping up a year-long investigation into the department's decisions before Democrats take the majority in January.

Comey first testified on December 7.

Australian Associated Press