U.S. President Trump responds to claims in "Fire and Fury" book

U.S. President Donald Trump on Saturday rejected an author's accusations that he is mentally unfit for office and said his track record showed he is "a very stable genius."

Michael Wolff, who was granted unusually wide access to the White House during much of Trump's first year, has said in promoting his new book that Trump is unfit for the presidency.

Trump, in a series of morning tweets, said his Democratic critics and the U.S. news media were bringing up the "old Ronald Reagan playbook and screaming mental stability and intelligence" since they have not been able to bring him down in other ways.

Reagan, a Republican who was the U.S. president from 1981-1989, was diagnosed with Alzheimer's disease in 1994 and died in 2004.

"Actually, throughout my life, my two greatest assets have been mental stability and being, like, really smart," said Trump.

"I went from VERY successful businessman, to top T.V. Star ... to President of the United States (on my first try). I think that would qualify as not smart, but genius ... and a very stable genius at that!"

Trump made the comments from the presidential retreat at Camp David, Maryland, where he was meeting Republican congressional leaders about their legislative agenda for the year.

Wolff's book, "Fire and Fury - Inside the Trump White House," has proved to be another shock to the system for Trump and his top aides, coming just as he starts his second year in office.

Wolff told BBC Radio in an interview broadcast on Saturday that he believed Trump is unfit for office.

He told NBC News on Friday that White House staff around Trump treated him like a child.

"The one description that everyone gave, everyone has in common — they all say he is like a child," Wolff said. "And what they mean by that, he has a 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."

Trump is to undergo at physical examination on Jan. 12, the first of his presidency.