Fired FBI Director James Comey has said there should be "no fuzz" about Russia's role in influencing the 2016 US election that brought Donald Trump to the White House, as he cited a "campaign driven from the top of that government" with purpose and technical sophistication.

"There should be no fuzz on this whatsoever. The Russians interfered in our election during the 2016 cycle," Comey said.

"They did it with purpose. They did it with sophistication. They did it with overwhelming technical efforts. It was an active-measures campaign driven from the top of that government," he told the Senate Select Committee on Intelligence during a much-anticipated hearing.

Comey, who was sacked in May, said this assessment was a "high-confidence judgment" of the entire American intelligence community. "It's not a close call. That happened," he said.

During his more than two hours of testimony yesterday, the 56-year-old former FBI chief said he was fired by Trump "because of the Russia investigation."

"I was fired in some way to change, or the endeavour was to change, the way the Russia investigation was being conducted," he told lawmakers at the Congressional hearing.

Comey said that soon after he was fired he leaked the information about his interaction with Trump hoping that such a move would prompt the appointment of a special counsel to investigate the Russsian links to the Trump campaign.

"I know I should've said this earlier, but it's obvious - if any Americans were part of helping the Russians do that to us, that is a very big deal. I'm confident that, if that is the case, Director (Robert) Mueller (who has been appointed as counsellor to investigate the Russian links) will find that evidence," Comey said in response to a question.

Referring to his own sacking, Comey said that the Trump administration had started a public campaign of "lies, plain and simple" to undercut his and the bureau's credibility.

"That is a very big deal, and not just because it involves me. The nature of the FBI and the nature of its work requires that it not be the subject of political consideration. On top of that the Russia investigation itself is vital, because of the threat," he said.

Comey also accused the White House of defaming him. His testimony does not implicate the Trump presidency, but it nonetheless questions the billionaire Republican president's personality.

Comey said he began documenting his meetings with Trump because of his concerns with the US president's "nature".

"I was honestly concerned he might lie about the nature of our meeting, so I thought it important to document," he said.

In April, the New York Times reported that Comey began jotting down his interactions with Trump after a January 6 meeting at Trump Tower, following which the former FBI Director felt Trump was apparently seeking favours.

(This article has not been edited by DNA's editorial team and is auto-generated from an agency feed.)