Everything you need to know about the former FBI chief's accusations on Donald Trump.

Former FBI director James Comey has accused President Donald Trump of firing him to try to undermine the bureau’s investigation into possible collusion between his 2016 presidential campaign team and Russia.

The former FBI head on Thursday also told senators at the Senate Intelligence Committee Trump’s administration defamed him in comments made after his May firing by saying the bureau was in disarray and the workforce had lost confidence in him.

“Those were lies, plain and simple,” Comey said.

Asked at the hearing why he was fired, Comey said he did not know for sure, but referred to the president’s words.

“I know I was fired because of something about the way I was conducting the Russia investigation was in some way putting pressure on him, in some way irritating him, and he decided to fire me because of that.”

Comey earlier testified he believed Trump directed him to drop an FBI probe into the Republican president's former national security adviser Michael Flynn as part of the Russia investigation.

Comey said he interpreted Trump's wording "I hope you can let this go" about the Flynn investigation as a direction.

But Comey would not say whether he thought the president sought to obstruct justice, calling the conversation "disturbing".

Trump critics say any efforts by the president to hinder an FBI probe could amount to obstruction of justice.

Such an offence could lead to Trump being impeached.

Dressed in a dark suit and giving short, deliberative answers, Comey painted a picture of an overbearing president who pressured him to stop the FBI looking into Flynn.

In more than two hours of testimony, Comey did not make any major new revelations about alleged links between Trump or his associates and Russia.

Russia has denied such interference and the White House has denied any collusion.

As Comey was testifying, Trump, in a speech across town, told supporters their movement was "under siege" and vowed to fight on.

Trump's son Donald Trump Jr fired off a series of tweets attacking Comey but the president himself stayed off social media.

Trump's personal lawyer, Mark Kasowitz, said afterwards Comey's testimony proved the president was not under any investigation and there is no evidence a single vote was changed as a result of Russian interference in last year's election.

Comey said he felt he needed to get his account of his conversations with Trump in the public sphere in the hope that it might prompt the appointment of a special counsel, which later occurred.

Comey said he showed copies of his memo memorialising his talks with Trump to people outside the Justice Department and "asked a friend of mine to share the content of the memo with a reporter".

Comey said he did not know if there are tapes of his conversations with Trump but said he hoped there were and that they should be made public.

"Lordy, I hope there are tapes," Comey said.