Washington (United States) (AFP) - Donald Trump fired back Monday at James Comey, saying he is guilty of "many crimes," as the former FBI director opened a book tour with an interview in which he labelled the US president "morally unfit" for office.

Trump accused Comey of lying to Congress and exonerating Hillary Clinton in a 2016 investigation because of her strong poll numbers in the presidential race.

"Comey drafted the Crooked Hillary exoneration long before he talked to her (lied in Congress to Senator G)," Trump fired in an early morning tweet.

"Then based his decisions on her poll numbers. Disgruntled, he, [former FBI deputy director Andrew] McCabe, and the others, committed many crimes!"

Late Sunday, Comey launched a publicity tour for his new memoir, "A Higher Loyalty: Truth, Lies and Leadership," with an ABC television interview in which he branded Trump a serial liar who will "stain everyone around him."

Comey's book, which leaked out last week ahead of its official release Tuesday, takes on the president who fired him in May 2017 over the troubling Russia election meddling investigation, a probe that poses a deep threat to Trump's 15-month-old presidency.

- Trump 'morally unfit' -

Comey is schedule to give interviews on the book to major television networks this week and travel to a dozen cities to promote the book in person.

"I think he's morally unfit to be president," Comey told ABC.

Trump "talks about and treats women like they're pieces of meat" and "lies constantly about matters big and small and insists the American people believe it."

Comey also said that serving in Trump's administration poses a serious ethical dilemma.

"The challenge of this president is that he will stain everyone around him," he told ABC.

"And the question is, how much stain is too much stain and how much stain eventually makes you unable to accomplish your goal of protecting the country and serving the country?"

- Republicans: 'Lyin' Comey' -

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Trump and the Republicans have sought to stifle the impact of his book by accusing Comey of leaking classified materials and of corruptly handling the 2016 investigation into Clinton.

The Republican Party set up a website to attack the book, branding him "Lyin' Comey."

Trump called Comey an "untruthful slime ball" and said it was his "great honor" to fire the veteran Justice Department prosecutor.

"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!" Trump said Sunday.

In a separate ABC interview, White House press secretary Sarah Sanders reiterated the charges of lying and divulging classified materials.

"Look, it's been very clear that James Comey is a self-admitted leaker. He lied to Congress," she said.

- Trump's legal troubles -

In his book, Comey likens Trump to a dishonest, ego-driven mob boss and says he demanded the then FBI chief's personal pledge of loyalty.

That damning account has infuriated the president at a moment of intensifying legal pressure on other fronts.

The probe into possible collusion between the Trump campaign and Russians during the 2016 presidential race, now led by independent special prosecutor Robert Mueller, increasingly menaces Trump's inner circle.

And last week federal agents in New York raided the office and hotel room of Trump's longtime personal lawyer and fixer Michael Cohen, possibly in connection with secret hush payments he made to women claiming they had affairs with Trump.

In his interview Sunday, Comey said Trump could be guilty of obstruction of justice in the way he tried to get the FBI to drop a probe of one of his staff in early 2017.

But Comey said he could not confirm suspicions that Moscow possesses compromising material -- an alleged lurid video of the president with prostitutes -- or evidence of collusion during the election that could be used to blackmail the US president

"I think it's possible. I don't know," Comey said.