US President Donald Trump has abruptly dismissed the director of FBI, saying that James 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 US presidential election and possible collusion with Trump's campaign.

His dismissal on Tuesday is likely to prompt concerns about the integrity of the inquiry and renew calls for an independent investigation.

Comey had been embroiled in a controversy surrounding his investigation into whether Hillary Clinton's use of a private email server while US secretary of state during Barack Obama's first term as president 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.

'Political bombshell'

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

"President Trump’s nearly four months in office have been controversial, but this is almost certainly the most controversial decision yet," said Al Jazeera's Washington Editor James Bays, calling the move "a political bombshell that will reverberate for a long time.

"It doesn't end here, because a new FBI director has to be confirmed by the Senate, where the Republicans hold a very slim majority."

Comey had said in July the Clinton email case should be closed without prosecution, but then declared - 11 days before the November 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 said last week that she partly blames Comey's decision for her election loss.

The White House released a memo by Rod Rosenstein, deputy attorney general, 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.

Chuck Schumer, the Democratic senator, said firing Comey was "a big mistake", and questioned the timing of the move.

"We know the FBI has been looking into whether the Trump campaign colluded with the Russians," he said.

"Were these investigations hitting too close to home for the president?"

Russia investigations

In his letter, Trump told Comey: "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 investigations ongoing in Congress.

The US House of Representatives’ main investigation has been blocked in recent weeks by squabbles, while the Senate’s parallel probe has been slow moving and equipped with a much smaller staff than previous prominent congressional investigations.

Richard Burr, the Senate Republican leading that investigation, said he was "troubled by the timing and reasoning" behind the firing.

He said he found Comey to a public servant of the highest order, and his dismissal "further confuses an already difficult investigation" by his panel.

Matthew Schmidt, an assistant professor of national security and political science at the University of New Haven, says Trump's move gives the impression that his administration has something to hide, as there have been no new revelations about the email inquiry to motivate the timing of the firing.

"Now there's no way that any report coming out of the FBI that is favourable to the president will be seen as being not tainted," he told Al Jazeera.

"The only way to save this is to appoint a special prosecutor that is seen as being independent of White House interference.

"The president's decision to remove the FBI director only confirms the need and the urgency of such a committee."

Some Republican politicians supported Trump's argument that the FBI needs a "fresh start".

Given the recent controversies surrounding the Director, I believe a fresh start will serve the FBI and the nation well. — Lindsey Graham (@LindseyGrahamSC) May 9, 2017

However, Republican Senator John McCain, a former presidential candidate, said he was "disappointed" by the dismissal of Comey, whom he called "a man of honour and integrity".

McCain said he had long called for a special congressional committee to investigate Russia's interference in the 2016 election.

Senator Bernie Sanders, Democratic presidential candidate in 2016, said Trump's decision to fire Comey "raises serious questions about what his administration is hiding".

"President Trump has repeatedly taken steps to kill inquiries into Russia's involvement in the US election. It is clear that whomever President Trump hand picks to lead the FBI will not be able to objectively carry out this investigation," he said in a statement.