Accusing the White House of defaming him with "lies", fired FBI director James Comey today said it was "very disturbing" that US President Donald Trump asked him to stop probing the Russia links of one of his top aide.

Comey said he saw Trump's request to stop investigating the alleged ties of his former national security adviser Michael Flynn as an "order".

Asked whether he considered Trump's request to drop the investigation of Flynn, Comey said, "I don't think it's for me to say whether the conversation I had with the president was an effort to obstruct. I took it as a very disturbing thing, very concerning."

The 56-year-old former FBI chief told the committee that he perceived Trump's comments over Flynn as an "order".

He also said Trump misguided the American public when he last month said the FBI was in "disarray". "Those were lies, plain and simple, and I'm so sorry that the FBI workforce had to hear them, and I'm so sorry the American people were told that," he said in his testimony to the Senate Intelligence Committee.

Asked about a dinner he had with Trump on January 27 - just days after his inauguration, Comey said the US president was trying to establish a "patronage relationship."

Describing that dinner, he said, "My common sense told me that what was going on is, either he thought or someone told him, 'you have already asked Comey to stay, and you did not get anything for it.' And the dinner was an attempt to build loyalty..."

"What the president whispered in my ear was, I really look forward to working with you...I'm sitting there thinking, three times we've already talked about me staying. My common sense told me, he's looking to get something in exchange..." Comey said.

When Senator Dianne Feinstein asked Comey why he didn't stop and say, "Mr president this is wrong" over the Flynn probe, Comey said "maybe if I were stronger I would have. I was so stunned by the conversation that I just took it in."

When he was asked how his colleagues reacted when Trump asked him to "let it go" (about the Flynn investigation), Comey said: "They were shocked," and "they were very concerned."

Responding to a question from Senator Burr, Comey said he has no doubt that the Russians attempted to interfere in the the 2016 elections.

Trump, Comey said, told him repeatedly that he had talked to lots of people about him, including his current attorney general, and had learned that he was doing a great job and that he was extremely well-liked by the FBI workforce.

"(But) on May 9, when I learned that I had been fired, for that reason, I immediately came home as a private citizen," he said.

"They confused me because the president and I had had multiple conversations about my job, both before and after he took office. And he had repeatedly told me I was doing a great job and he hoped I would stay.

"I had repeatedly assured him that I did intend to stay and serve out the remaining six years of my term," he told the lawmakers at the hearing being watched live across the US.

Comey was fired on May 9, when Trump expressed his deep frustration with the continuing probe into whether members of his presidential election campaign had tied up with Russian hackers against the billionaire tycoon's rival, Democrat Hillary Clinton.