US President Donald Trump ignited a political firestorm on Tuesday by firing Federal Bureau of Investigation (FBI) Director James Comey, who had been leading an investigation into the Trump 2016 presidential campaign's possible collusion with Russia to influence the election outcome.

The Republican president said he fired Comey, the top US law enforcement official, over his handling of an election-year email scandal involving then-Democratic presidential nominee Hillary Clinton.

The move stunned Washington and raised suspicions among Democrats and others that the White House was trying to blunt the FBI probe involving Russia.

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

White House officials denied allegations that there was any political motive in the move by Trump, who took office on January 20.

Senate Democratic leader Chuck Schumer said he spoke to Trump and told him he was "making a very big mistake" in firing Comey, adding the president did not "really answer" in response.

An independent investigation into Moscow's role in the election "is now the only way to go to restore the American people’s faith," Schumer said.

Though many Democrats have criticised Comey's handling of the Clinton email probe, they said they were troubled by the timing of Trump's firing of him.

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Senator Richard Burr, the Republican chairman of the Senate Intelligence Committee, which is overseeing its own investigation into Russian interference during the election, said in a statement he was also troubled by the timing of Comey's termination.

"His dismissal, I believe, is a loss for the Bureau and the nation," Burr said.

US intelligence agencies concluded in a January report that Russian President Vladimir Putin had ordered an effort to disrupt the 2016 election, with the aim of helping Trump.

CNN reported on Tuesday night that federal prosecutors had issued grand jury subpoenas to former Trump national security adviser Michael Flynn, seeking business records, as part of the probe into Russian interference in the election.

Trump's firing of Comey came a day after former acting Attorney General Sally Yates told a Senate panel that she had informed the White House on January 26 that Flynn was at risk of blackmail by Moscow because he had been untruthful about his discussions with the Russian ambassador, Sergei Kislyak. Trump fired Flynn 18 days later.

Russia has repeatedly denied any meddling in the election and the Trump administration denies allegations of collusion with Russia.

Restoring 'public trust'

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."

The president told Comey in the letter that 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. He was appointed director by Democratic President Barack Obama in 2013.

Sessions advised Trump's campaign before being picked by the president to lead the Justice Department. Sessions had recused himself from involvement in the Russia investigation, after he misstated his own 2016 contacts with Russia's ambassador to Washington.

Comey's deputy, Andrew McCabe, became acting FBI director.

The White House said the search for a new FBI director would begin immediately.

Trump, in the letter, 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."

Email controversy

Comey, 56, had been the target of criticism from many quarters for his handling of a probe involving Clinton's use of a private email server while she was US secretary of state under Obama. As recently as Tuesday, the FBI clarified remarks that Comey made on the matter last week.

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 help cost her the election.

The firing came as a shock to FBI staff, nearly all of whom had confidence in Comey despite the controversy surrounding his handling of the Clinton email situation, according to an FBI official who spoke on condition of anonymity.

The official said there was concern among agents that the firing was a political act related to the Russian investigation.

Other current and former US intelligence and law enforcement officials questioned the White House explanation for Comey's firing.

"Trump praised him for the work on the email investigation, so that’s not it," said Austin Berglas, a former FBI supervisory agent on hacking cases. "I think he realized the extent of the Russia investigation under way and moved him out. To me, that’s the only logical explanation right now."

Trump’s dismissal of Comey does not mean the FBI’s investigation into Russia’s interference in the US presidential election will be disrupted or end — career FBI staffers can continue the probe even as the search for a new FBI director begins, legal experts said.

Republican Representative Justin Amash wrote on Twitter that he and his staff were reviewing the possibility of drafting legislation to create an independent commission to look into the Russian campaign meddling.

Legislation related to the appointment of a special prosecutor or independent counsel has lapsed. But Justice Department regulations provide for the appointment of a special counsel, which is selected by the attorney general, or acting attorney general in the case of recusal, the experts said.