You're fired! Comey sacking shocks US politics

US President Donald Trump fired FBI director James Comey late Tuesday in a shock move which drew immediate comparisons with the Watergate scandal that brought down Richard Nixon.

Comey was heading a wide-ranging investigation into whether Trump aides colluded with Russia to sway last year's election, with Democrats claiming allegations of a cover-up and demanding an independent inquiry.

The top Democrat in the Senate, Chuck Schumer, said Trump had made a "big mistake."

"This does not seem a coincidence," he said.

Unless the administration appoints an independent special prosecutor to probe the Russian meddling, Schumer added, "every American will rightly suspect that the decision to fire director Comey was part of a cover-up".

Democrat Senator Patrick Leahy of Vermont said Trump's decision was "absurd", describing it as "nothing less than Nixonian."

"That fig leaf explanation seeks to cover the undeniable truth: The president has removed the sitting FBI director in the midst of one of the most critical national security investigations in the history of our country - one that implicates senior officials in the Trump campaign and administration," he charged.

Trump's decision to fire the FBI director is virtually unprecedented in US history, with only one director previously fired in the bureau's century-long operations.

In a letter circulated by the White House, Trump told Comey: "You are hereby terminated and removed from office, effective immediately."

"It is essential that we find new leadership for the FBI that restores public trust and confidence in its vital law enforcement mission."

His dismissal was allegedly due to the mishandling of a probe into Hilary Clinton's emails, but the snap decision sparked rare criticism from Republicans as well.

"I am troubled by the timing and reasoning of Director Comey's termination," said Senator Richard Burr.

Congressman Justin Amash, a fellow Republican, described parts of Trump's letter to Comey as "bizarre" and announced "my staff and I are reviewing legislation to establish an independent commission on Russia".

The White House said the search for a new FBI director was to begin right away. FBI directors are appointed for a single 10-year term and 56-year-old Comey was appointed four years ago.

Trump's initial decision to keep Comey, who was appointed by Barack Obama, drew accusataions from critics that he was being tacitly rewarded for his role in damaging Clinton's chances at the elections.

But within months of the US election, Comey took aim at Trump, plainly rejecting his inflammatory claim that he was wiretapped by predecessor Obama.