James Comey has accused Donald Trump of lying to the American public when he sought to justify his dismissal as FBI director in stunning testimony that fuels claims of obstruction of justice against the President.

In his much-anticipated appearance before the Senate Intelligence Committee on Friday morning (AEST), Mr Comey denied Mr Trump’s claims the FBI was in disarray and that he had lost the confidence of agents.

“Those were lies, plain and simple,” Mr. Comey said in his opening remarks ahead of three hours of grilling.

And when asked if he had any doubts that Russia attempted to influence the 2016 US election, Mr Comey’s simple answer was “no”, later adding that Russia is “coming after America”.

Comey on Russians: "It's not about Republicans or Democrats. They're coming after America" https://t.co/cX5HuKjFfG https://t.co/JLL7BlZAgk

— The Lead CNN (@TheLeadCNN) June 8, 2017

While Mr Comey did not draw specific legal conclusions as to a potential obstruction of justice case against the President, he said Mr Trump’s request to terminate the FBI investigation into former national security adviser Michael Flynn left him “stunned”, and senior FBI officials considered it to be of “investigative interest”.

Mr Comey said of particular concern was the fact that President Trump asked other officials to leave the Oval Office before telling the then-FBI director: “I hope you can let this go”.

“Why did he kick everybody out of the Oval Office?” Mr Comey asked the committee. “That, to me as an investigator, is a very significant fact.”

The FBI is one of the bodies investigating potential connections between Mr Trump’s 2016 election campaign and allegations that Russia attempted to influence the outcome of the election.

Some in the US Congress have suggested President Trump’s dismissal of Mr Comey amounted to an attempt to obstruct justice.

Former FBI boss Bob Mueller has been appointed as a special counsel to oversee the Russia investigation.

Michael Flynn was sacked as National Security Adviser after it emerged that he lied to Vice President Mike Pence over meetings he held with Russian officials.

Mr Comey acknowledged under questioning that the President did not give him an order to drop the probe into Mr Flynn, but said he took the President’s remarks as a command.

“I took it as a direction,” Mr Comey said. “This is the President of the United States, with me alone, saying ‘I hope this.’”

Mr Comey confirmed that Mr Flynn was facing a criminal investigation at the time he was fired.

While backing up Mr Trump’s assertions that the President was not under direct investigation over potential Russian links, Mr Comey said Mr Trump’s conduct fell within “the scope of” the FBI probe.

Mr Comey admitted he was responsible for releasing his damning memos that outlined private discussions with the President and has since turned them over to Mr Mueller.

The former FBI director confirmed he detailed every interaction with the President because he feared Mr. Trump might lie about what was said.

“I turned them over to Bob Mueller’s investigators,” he said of his notes.

Mr. Comey testified that he considered it very unusual for him to be discussing ongoing investigations, alone, with a sitting president.

He said he would welcome President Trump releasing tapes of the conversations, which he has said he holds.