US Secretary of State Rex Tillerson says he has “never questioned” President Trump’s mental health after the author of controversial book said his staff saw him as a “child”.
Michael Wolff says Fire and Fury: Inside the Trump White House is based on about 200 interviews.
The book has now gone 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: “They should try winning an election. Sad!”
Mr Tillerson – who reportedly called Mr Trump a “moron” earlier this year – told CNN: “I have no reason to question his mental fitness.”
He said Mr Trump was “not typical of presidents of the past”.
“I think that’s well recognized. That’s also though why the American people chose him,” he said.
Mr Tillerson added that he had a “developing relationship” with the president and denied speculation that he was set to quit the White House after a rumoured rift, saying he would serve for the whole of 2018.
“In terms of what I would do different, I’m going to build on my ability to communicate with the president better,” he told CNN.
On Friday, Mr Trump refused to answer questions about the book as he departed for Camp David from the White House. Mr Trump will spend two days meeting top Republicans at the retreat to discuss his legislative priorities for the year ahead.
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 three hours with Mr Trump in total, both during the election campaign and after the inauguration.
Hitting back in a television interview at the White House attacks, he said the president had no credibility and that “100% of the people around him” question his fitness for office.
Wolff said it was “extraordinary” that the president of the US would try to stop publication of his book, a move that “the CEO of a mid-sized company” would not attempt.
The book cites former top aide Steve Bannon as describing a meeting at Trump Tower in New York 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 Russians.
However, the accuracy of some excerpts has been criticised and questioned in the 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, BBC North America editor Jon Sopel says. –BBC