US Secretary of State Rex Tillerson says he has never doubted President Trump's mental health after a new book claimed staff saw him as a child.

Author Michael Wolff said White House employees believed Mr Trump's "mental powers were slipping".

His book, Fire and Fury: Inside the Trump White House, went on sale early despite the president's attempts to block its publication.

Mr Trump says the book is "boring and untruthful" and Wolff a "total loser".

He said it was being pushed by the media and others to hurt him. He added in a tweet: "They should try winning an election. Sad!"

Mr Tillerson - who is alleged to have called Mr Trump a moron last year - told CNN: "I have no reason to question his mental fitness."

Rex Tillerson

He said Mr Trump was "not typical of presidents of the past".

"I think that's well recognised. That's also though why the American people chose him," he said.

What are the questions on Trump's mental health?

In a television interview on Friday, Wolff said "100% of the people" around Mr Trump questioned his fitness for office.

His book alleges that Mr Trump failed to recognise close friends, and was prone to repeating comments.

Wolff said that White House staff described the president as childlike because "he has the need for immediate gratification. It's all about him... This man does not read, does not listen. He's like a pinball just shooting off the sides."

The president said he had not given Wolff access to the White House nor spoken to him for the book.

But Wolff responded: "What was I doing there if he didn't want me to be there? I absolutely spoke to the president... It was not off the record."

He said he had spent a total of three hours with Mr Trump, both during the election campaign and after the inauguration.

What else is in the book?

It cites former top aide Steve Bannon as describing a meeting between a Russian lawyer and Trump election campaign officials, including Mr Trump's son Donald Jr, as "treasonous".

Both Mr Trump Jr and his father deny that any collusion with Russians to win the election took place. However Mr Bannon is quoted in the book as saying: "They're going to crack Don Junior like an egg on national TV."

The meeting is being investigated by Special Counsel Robert Mueller as part of his inquiry into possible collusion between Trump campaign officials and Russia.

The book makes many other claims, including that:

• The Trump team was shocked and horrified by his election win
• His wife, Melania, was in tears of sadness on election night
• His daughter, Ivanka, had a plan with her husband, Jared Kushner, that she would be "the first woman president"
• Ivanka mocked her dad's "comb-over" hairstyle and "often described the mechanics behind it to friends"

The accuracy of some excerpts has been criticised and questioned in US media.

Still, even if only half of what the book contains is true, it paints a damning portrait of a paranoid president and a chaotic White House, says BBC North America editor Jon Sopel.

Will it actually hurt Trump?

Analysis: BBC North America reporter Anthony Zurcher

Donald Trump's supporters have seen many a media storm over the past few years, and somehow their man always emerges (relatively) unscathed. The book may be generating considerable heat among the chattering class, but there's little to indicate that its lasting impact will be much more than confirming long-held suspicions of Trump critics and re-enforcing the bunker mentality in the White House.

Outside Washington, in places where people don't devoutly follow every permutation of the presidential Twitter feed, the Trump administration is compiling a boast-worthy economic record.

Despite some doom-and-gloom predictions following the 2016 election, the stock market has soared. Unemployment remains low. Major corporations are making high-profile moves to at least temporarily boost their workers' paycheques. And the president can start pointing to his party's tax bill as a tangible reason why the economy is humming along.

If the current trajectory continues, Mr Trump and his fellow Republicans will be positioned to make the case to voters in the months and years ahead that despite all the drama - the often self-inflicted fire and fury - their agenda is to help Americans where it counts the most, in their pockets.

That's the kind of message that can win.