Sacked FBI director James Comey has described Donald Trump as "untethered to truth" and likened him to a mafia boss in a new book expected to spark fresh turmoil in the White House.

According to excerpts of the book leaked by US media on Thursday, Mr Trump was also obsessed with the alleged existence of a video with Russian prostitutes.

In the book to be released officially next Tuesday, Mr Comey, whom Mr Trump fired in May 2017, says the US president lives in "a cocoon of alternative reality" that he tried to pull others around him into, according to The Washington Post.

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Meetings with Mr Trump gave Mr Comey "flashbacks to my earlier career as a prosecutor against the Mob," he writes.

"The silent circle of assent. The boss in complete control. The loyalty oaths. The us-versus-them worldview. The lying about all things, large and small, in service to some code of loyalty that put the organisation above morality and above the truth."

But Mr Comey goes farther to say that Mr Trump, congenitally, has no sense of what is right and wrong.

"This president is unethical, and untethered to truth and institutional values," he writes, according to The New York Times.

"His leadership is transactional, ego driven and about personal loyalty."

'A Higher Loyalty'

The book, entitled A Higher Loyalty: Truth, Lies and Leadership, has triggered worries now rippling through the White House and Republican establishment over the damage it might do to the already deeply troubled Mr Trump presidency.

The White House has sought to cast doubts about Mr Comey's reputation: the Republican Party this week put up a website called "Lyin' Comey" to undermine the book.

But among both Republicans and Democrats in Washington's establishment, the reputation of Mr Comey - who has served in the FBI under three presidents - is much stronger than his book's target.

Last year Mr Comey revealed that Mr Trump had pressured him to drop an investigation into Mike Flynn, the US leader's former national security advisor, and also demanded a loyalty pledge.

Getting neither, the president fired Mr Comey on May 9, complaining of the ongoing investigation into possible Trump campaign-Russia ties.

A week later the Justice Department appointed a special prosecutor Robert Mueller, who has doggedly pursued the probe as well as possible obstruction of justice by Trump. He has issued 19 indictments so far, including of top Trump lieutenants.

But "A Higher Loyalty" shares little new about the Russia investigation, with Comey bound to respect the classified nature of the Mueller investigation that Trump just this week labeled "fake & corrupt."

Alleged video

Mr Trump's concern over the alleged video extended to the numerous sex scandals that have dogged him over decades as a New York real estate tycoon - and have returned to haunt him in the White House.

In the past year two women, one a pornographic film star and the second a Playboy model, have said they were paid off to be silent about affairs they had with Mr Trump over a decade ago while he was married to his current wife Melania.

Speaking on the Moscow video, Mr Comey said, Mr Trump "strongly denied the allegations, asking - rhetorically, I assumed - whether he seemed like a guy who needed the service of prostitutes."

"He then began discussing cases where women had accused him of sexual assault, a subject I had not raised," Mr Comey wrote. "He mentioned a number of women, and seemed to have memorised their allegations."