US President Donald Trump has unleashed a torrent of rage against former FBI Director James Comey, calling him “slippery”, insisting he never asked him for loyalty and labelling him “the worst FBI Director in history, by far!”

Mr Trump fired off a series of furious tweets early ahead of an interview Mr Comey conducted with US television network the ABC that was set to air in full on Sunday evening local time.

In an excerpt shown on Saturday, Mr Comey said his belief that Hillary Clinton would win the 2016 presidential election was a factor in his decision to disclose the investigation into her emails.

Mr Trump seized on that, saying Mr Comey “was making decisions based on the fact that he thought she was going to win, and he wanted a job. Slimeball!”

President Trump also pushed back again against Mr Comey’s claims that Mr Trump sought his loyalty, saying it was “Just another of his many lies. His ‘memos’ are self serving and FAKE!”

I never asked Comey for Personal Loyalty. I hardly even knew this guy. Just another of his many lies. His “memos” are self serving and FAKE!

— Donald J. Trump (@realDonaldTrump) April 15, 2018

Mr Comey is embarking on a public rollout of his book, A Higher Loyalty, which comes out on Tuesday.

The book offers Mr Comey’s version of the highly controversial events surrounding his firing by Mr Trump and the investigations into Russian election meddling and Ms Clinton’s email practices.

In it, Mr Comey compares Mr Trump to a mafia don and calls his leadership of the country “ego driven and about personal loyalty”.

Mr Trump’s tirade continued on Monday morning (AEDT), calling “slippery James Comey” the “worst FBI Director in history”.

Slippery James Comey, a man who always ends up badly and out of whack (he is not smart!), will go down as the WORST FBI Director in history, by far!

— Donald J. Trump (@realDonaldTrump) April 15, 2018

President Trump fired Mr Comey in May 2017, setting off a scramble at the Justice Department that led to the appointment of Robert Mueller as special counsel overseeing the Russia investigation.

Mr Mueller’s probe has expanded to include whether Mr Trump obstructed justice by firing Mr Comey, which the president denies.

Mr Trump has said he fired Mr Comey because of his handling of the FBI’s investigation into Ms Clinton’s email practices. Mr Trump used the investigation as a cudgel in the campaign and repeatedly said Ms Clinton should be jailed for using a personal email system while serving as secretary of state.

Democrats, on the other hand, have accused Mr Comey of politicising the investigation, and Ms Clinton herself has said it hurt her election prospects.

In the interview excerpt released on Saturday, Mr Comey said he did not remember “consciously thinking” about the election results as he decided to disclose that the FBI had reopened its investigation into candidate Ms Clinton’s email use, but he said “I was operating in a world where Hillary Clinton was going to beat Donald Trump, and so I’m sure that it was a factor.”

He added: “I don’t remember spelling it out, but it had to have been, that she’s going to be elected president and if I hide this from the American people, she’ll be illegitimate the moment she’s elected, the moment this comes out.”

-AP