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.”
“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.”