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 "full of lies".
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.