Donald Trump is "morally unfit" to be president of the United States, former FBI director James Comey told ABC in an interview broadcast last night.

"I don't buy this stuff about him being mentally incompetent or early stages of dementia," Mr Comey said of President Trump, according to a transcript from ABC.

"I don't think he's medically unfit to be president. I think he's morally unfit to be president."

"Our president must embody respect and adhere to the values that are at the core of this country. The most important being truth. This president is not able to do that," he said.

Mr Trump fired Mr Comey in May 2017, citing the FBI's behaviour in investigating Democrat Hillary Clinton and its probe into possible Trump campaign collusion with Russia to tilt the 2016 presidential election.

Russia has denied interfering in the election and President Trump has denied any collusion or improper activity.

Just 11 days before the election, Mr Comey announced that the FBI would reopen an investigation into Mrs Clinton's possible misuse of a private email server while she was secretary of state - a move she has said played a part in her loss to Mr Trump.

The president lashed out at Mr Comey in a series of tweets earlier yesterday, saying that the email probe was handled "stupidly" and calling Mr Comey a "slime ball."

Mr Comey said in the exclusive interview with ABC News' George Stephanopoulos, that the US leader is doing "tremendous damage" to institutional and cultural norms.

He also said he was worried the president may be open to blackmail by Russia over alleged incidents involving prostitutes in Moscow in 2013.

Mr Comey said it is "possible, but I don't know" whether Russia has evidence to back up the allegations about Mr Trump's Moscow trip.

Mr Trump told Mr Comey that he had not stayed overnight in the Moscow hotel and that the claims related to the prostitutes were not true, Mr Comey said.

Mr Comey has a tell-all book, 'Higher Loyalty,' due out on tomorrow.

Transcript: James Comey's interview with ABC News
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The book's imminent release - and the slated ABC News interview - prompted Mr Trump to hurl a new set of insults at Mr Comey earlier, challenging accusations made in the book, and insisting that he never pressed Mr Comey to be loyal to him.

"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!" Mr Trump wrote early yesterday in one of five Twitter posts aimed directly at Mr Comey.

Reuters and other news outlets have obtained copies of Mr Comey's book before its formal release. In it, Mr Comey wrote that Mr Trump, in a private meeting, pressed the then FBI director for his loyalty.

Mr Comey told ABC News that the title of the book came from that "bizarre conversation" he had with Mr Trump at the White House in January 2017, shortly after his inauguration.

"He asked for my loyalty personally as the FBI director. My loyalty's supposed to be to the American people and to the institution," Mr Comey said in the interview.

The FBI has long tried to operate as an independent law enforcement agency.

"I never asked Comey for Personal Loyalty. I hardly even knew this guy. Just another of his many lies," Mr Trump said on Twitter.

Mr Comey is now a crucial witness for Special Counsel Robert Mueller's investigation into whether President Trump has tried to obstruct the Russia probe.