Donald Trump defends himself as a "very stable genius".

Donald Trump has rejected an author’s accusations that he is mentally unfit for office and said his track record showed he is a “a very stable genius.”

The US president hit back after an explosive account of life at the White House claimed he never intended to enter the Oval Office.

Author Michael Wolff also said in promoting his book, Fire and Fury – Inside the Trump White House, that Trump is unfit for the presidency.

But Trump, in a series of extraordinary morning posts on Twitter, said his Democratic critics and the US 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 US 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.

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“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’s tweets showed his frustration at what he views as unfair treatment by the news media of his presidency amid a federal investigation into whether he or his campaign aides colluded with Russia during the 2016 presidential campaign.

Wolff told BBC Radio on Saturday that based on his interviews with the people around Trump that he believed the president was unfit for office.

“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.”

Trump, answering questions from reporters at Camp David, called Wolff a “fraud” and said the book is “a complete work of fiction.”

“I think it’s a disgrace,” he said.

Trump said he never granted Wolff an interview for the book and blamed former adviser Steve Bannon, who he called “Sloppy Steve,” for granting Wolff access at the White House.

As the book shot to the top of the online bestseller lists in the US, White House spokeswoman Sarah Huckabee Sanders continued to dismiss the claims of a dysfunctional presidency which have gripped the American public.

Speaking on US television on Friday, she said that Mr Wolff had never interviewed Mr Trump despite having “repeatedly begged to see the president.”

Mr Wolff however insisted that he “absolutely” spoke to the president, adding whether he “realised it was an interview or not, I don’t know, but it was certainly not off the record.”