U.S. President Donald Trump on Tuesday abruptly fired FBI Director James Comey in the fallout over Comey's probe of Democrat Hillary Clinton's emails last year, saying Comey was no longer able to effectively lead the law enforcement agency.

Comey had been leading an FBI investigation into allegations of Russian meddling in the 2016 U.S. presidential election and possible collusion with Trump's campaign. His dismissal will likely fuel concerns about the integrity of the probe and renew calls for an independent investigation.

The FBI director had been embroiled in a controversy surrounding his probe into whether Clinton's use of a private email server while U.S. secretary of state during President Barack Obama's first term compromised national security.

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

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.

The decision, announced by White House press secretary Sean Spicer in a brief appearance before reporters, caught Washington off guard.

Comey had said in July the Clinton email case should be closed without prosecution, but then declared - 11 days before the Nov. 8 election in which Clinton was the Democratic nominee - 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 help 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 now that Comey has been fired.

"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."

There are several Russia probes ongoing in Congress. The U.S House of Representatives’ main investigation has been stymied in recent weeks by partisan squabbles, while the Senate’s parallel probe has been slow-moving and equipped with a much smaller staff than previous high-profile congressional investigations.