Updated 12.24pm US President Donald Trump has abruptly fired FBI Director James Comey, who had been leading his agency's investigation into alleged Russian meddling in the 2016 US presidential campaign and possible collusion with Trump's campaign.

Trump said the sacking, which shocked Washington, resulted from Comey's handling of an election-year email scandal last year involving then-Democratic presidential nominee Hillary Clinton. Democrats immediately accused Trump of acting out of political motives.

Comey had been embroiled in a controversy surrounding his handling of a probe of Clinton's use of a private email sever while she was US secretary of state. As recently as Tuesday, the FBI clarified remarks that Comey made on the matter last week.

Trump, in a letter to Comey released by the White House, said: "It is essential that we find new leadership for the FBI that restores public trust and confidence in its vital law enforcement mission."

Trump told Comey in the letter he accepted the recommendation of Attorney General Jeff Sessions that he could no longer provide effective leadership. Comey's term was to run through September 2023. Sessions advised Trump's campaign before being picked by the president to lead the Justice Department.

Statement from @WhiteHouse @PressSec on @FBI Director pic.twitter.com/EdBRntMim5

— Sean Spicer (@PressSec) May 9, 2017

Democrats denounced Trump's move, which some compared to the "Saturday Night Massacre" of 1973, in which President Richard Nixon fired an independent special prosecutor investigating the Watergate scandal.

"Today's action by President Trump completely obliterates any semblance of an independent investigation into Russian efforts to influence our election, and places our nation on the verge of a constitutional crisis," said Representative John Conyers, senior Democrat on the House Judiciary Committee.

Conyers and other Democrats renewed their calls for an independent commission or a special prosecutor to investigate Russian influence in the 2016 election.

Trump had originally criticised the FBI director for not pursuing criminal charges against Clinton last July, but later lavished praise on him.

Comey had said in July the Clinton email case should be closed without prosecution, but then declared - 11 days before the November 8 election - that he had reopened the investigation because of a discovery of a new trove of Clinton-related emails.

Clinton and other Democrats say they believe Comey's decision cost her the election.

The White House released a memo by Deputy Attorney General Rod Rosenstein that provided the administration's justification for firing Comey.

"I cannot defend the Director's handling of the conclusion of the investigation of Secretary Clinton's emails, and I do not understand his refusal to accept the nearly universal judgment that he was mistaken," Rosenstein wrote.

Senate Democratic Whip Dick Durbin went to the Senate floor on Tuesday to urge the White House to clarify whether the FBI investigation of Russian interference in the presidential campaign would continue.

"Any attempt to stop or undermine this FBI investigation would raise grave constitutional issues," Durbin said. "We await clarification by the White House as soon as possible as to whether this investigation will continue."

Trump, in his letter to Comey, said: "While I greatly appreciate you informing me, on three separate occasions, that I am not under investigation, I nevertheless concur with the judgment of the Department of Justice that you are not able to effectively lead the bureau."

Wow. letter from the WH notifying Director Comey of his immediate dismissal from office. pic.twitter.com/PXv5ILL5QH

— Pwn All The Things (@pwnallthethings) May 9, 2017

And letter from AG sessions recommending his removal pic.twitter.com/eTqdYMZOYP

— Pwn All The Things (@pwnallthethings) May 9, 2017

And D AG Rosenstein memo recommending Comey's dismissal. pic.twitter.com/cWXgmecT6k

— Pwn All The Things (@pwnallthethings) May 9, 2017