Former FBI director James Comey has accused the US President of undermining the rule of law in the United States by lying about the FBI, and he has urged Republican politicians to "stand up and speak the truth" about Donald Trump's behaviour.

Key points: James Comey said Congressional committees questioned him about Hillary Clinton's emails

James Comey said Congressional committees questioned him about Hillary Clinton's emails Comey said Republicans were ignoring ongoing investigations into Trump

Comey said Republicans were ignoring ongoing investigations into Trump Comey said FBI's reputation suffers because President "lies about it constantly"

Mr Comey — who was fired by Mr 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.

Mr Comey said politicians had again asked about former secretary of state Hillary Clinton's emails and a dossier that Republicans claim was used to justify a warrant to conduct secret surveillance of a Trump presidential campaign aide.

"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," Mr Comey told reporters after spending more than five hours being interviewed behind closed doors by the House Judiciary and House Oversight committees.

"How does that make any sense at all?

"Republicans used to understand that the actions of a president matter, the words of a president matter, the rule of law matters and the truth matters. Where are those Republicans today?"

Mr Trump has called the Russia investigation by Special Counsel Robert Mueller a "witch hunt" and on Sunday on Twitter labelled his own former personal lawyer, Michael Cohen, a "rat" for cooperating with prosecutors.

He also accused FBI agents of breaking into Mr Cohen's office when they were in fact acting with a search warrant.

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Mr Cohen was sentenced to three years' prison last week for crimes including orchestrating hush payments to women in violation of campaign laws before the 2016 election.

Mr Cohen said he was directed by Mr Trump.

"The FBI's reputation has taken a big hit because the President of the United States, with his acolytes, has lied about it constantly," Mr Comey said when asked whether he bore any responsibility for damaging the FBI's reputation.

"At some point, someone has to … stand up for the values of this country and not slink away in retirement, but stand up and speak the truth."

Asked about the President calling Mr Cohen a "rat", Mr Comey said it undermined the rule of law.

White House Press Secretary Sarah Sanders responded 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."

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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.

Mr Comey first testified on December 7.

A transcript released after Mr Comey's first interview showed a heavy focus on Ms Clinton's email probe. A transcript of the second interview will also be released.

Reuters/AP