Former FBI Director James Comey has called the Trump administration's shifting explanations for firing him "lies, plain and simple" and said he wrote detailed memos of his conversations with President Donald Trump because he feared the president would paint a false picture of their encounters.

Mr Comey began giving testimony to the Senate Intelligence Committee just after 10am local time (midnight AEST), for what is the most widely anticipated US congressional hearing in years. 

The Senate panel released Mr Comey's written testimony on Wednesday, in which Mr Comey quoted Mr Trump as telling him the Russia investigation was a "cloud" impairing his ability to operate as president.

In a one-on-one meeting in the Oval Office on February 14, Mr Comey's statement said, Mr Trump asked him to drop an investigation into former national security adviser Michael Flynn, part of a wider probe into Russian meddling in the election.

"I hope you can see your way clear to letting this go, to letting Flynn go," Mr Comey quoted Mr Trump as saying.

Mr Comey elected not to read that testimony aloud on Thursday, instead making a separate brief statement.

He said prior to his firing he had been assured by the president he was doing a good job, and that the later explanations for his firing left him confused.

"When I was appointed FBI director in 2013, I understood that I served at the pleasure of the president," Mr Comey told the Senate Intelligence Committee.

"On May the ninth, when I learned that I was fired, for that reason, I immediately came home as a private citizen," he said. "But then the explanations, the shifting explanations confused me and increasingly concerned me."

He said that though no explanation was required for firing an FBI director "the administration chose to defame me and more importantly the FBI, by saying the organisation was in disarray, that it was poorly led, that the workforce had lost confidence in its leader."

"Those were lies, plain and simple, and I am so sorry that the FBI workforce had to hear them and that the American people were told that," Mr Comey said.

Asked why he took detailed notes about his meetings with the president, Mr Comey said "I was honestly concerned he might lie about the nature of our meeting, so I thought it really important to document... it led me to believe I gotta write it down."

Lawmakers are scrutinising Mr Comey's account to determine whether Mr Trump crossed any legal boundaries. Mr Comey, 56, declined to say whether the president's request to end the Michael Flynn probe constituted obstruction of justice, saying he would leave a determination about that to Robert Mueller, another former FBI chief who is now special counsel leading the Russia probe and who is now in possession of Mr Comey's memos.

While Mr Comey told senators he found the discussion with Mr Trump "a very disturbing thing," he added that "I don't think it's for me to say whether the conversation I had with the president was an effort to obstruct."

At one point in the hearing, Republican Senator James Risch pressed Mr Comey on whether Mr Trump had actually directed him to drop the Flynn investigation.

"He did not order you to let it go?" Mr Risch asked.

"Again, those words are not an order," Mr Comey said. "I took it as a direction. If it is the president of the United States, with me alone, saying 'I hope this' - I took it as 'this is what he wants me to do.' I didn't obey that, but that's the way I took it," Mr Comey said

When asked later on whether he took Mr Trump's comments as a directive, Mr Comey, "Yes."

In a lighter moment, Mr Comey referred to Mr Trump's apparent warning on Twitter that there could be recordings of their conversations that would undercut the former FBI chief's account of events.

"Lordy, I hope there are tapes," Mr Comey said. But he later said Mr Trump's comment prompted him to ask an acquaintance to leak portions of the memos he prepared on his conversations with the president in an effort to spur the appointment of a special counsel.

While President Trump has so far stayed off Twitter during Mr Comey's testimony, his eldest son hasn't.

Donald Trump Jr has tweeted repeatedly during the closely watched testimony Thursday.

He repeatedly defended his father and attacked Mr Comey. Mr Trump Jr in particular seized on Mr Comey's assertion that he interpreted the president's statement that he "hoped" the FBI would drop its probe into former National Security Adviser Michael Flynn.

President Trump's personal attorney Marc Kasowitz plans to make a statement following Mr Comey's testimony.​

Mr Kasowitz's remarks are expected on Thursday afternoon in downtown Washington. Mr Trump tasked Mr Kasowitz late last month with responding to matters arising from various probes of Russian interference in the election. This would be the first public appearance by Mr Kasowitz.

According to a person close to the president's legal team, Mr Trump will dispute key parts of Mr Comey's testimony.

Chairman of the committee, Republican senator Richard Burr, opened the hearing by telling the former FBI director this was his chance "to set the record straight", before reading a long opening statement of his own.

"We will establish the facts separate from rampant speculation and lay them out before the American people to make their own judgment," Mr Burr said. "Today is your opportunity to set the record straight. The American people need to hear your side of the story just as they need to hear the president's descriptions of events."

The Senate panel's top Democrat, Senator Mark Warner, said in his opening statement that Mr Comey's testimony showed Mr Trump violated guidelines put in place after the 1970s Watergate scandal to prevent White House interference in FBI investigations.

"I do want to emphasise what is happening here - the president of the United States is asking the FBI director to drop an ongoing investigation into the president's former national security adviser," Mr Warner said. 

The outcome could have significant repercussions for Mr Trump's presidency as special counsel Robert Mueller and several congressional committees investigate alleged Russian efforts to influence the 2016 presidential election and whether Mr Trump's campaign colluded with this. Russia has denied such interference and the White House has denied any collusion.

The issue has dogged Mr Trump's first five months in office, with critics saying that any efforts by him to hinder the FBI probe could amount to obstruction of justice.

This story will be continuously updated with news from the hearing.

Agencies and Fairfax Media