Tillerson defends president as book casts doubt on Trump’s mental health

Washington's chief diplomat Rex Tillerson found himself compelled to defend Donald Trump's fitness for office on Friday after a bombshell new book called the president’s mental health into question.

In an extraordinary portion of a television interview on foreign policy challenges, Tillerson was asked about claims that Trump has a short attention span, regularly repeats himself and refuses to read briefing notes.

"I've never questioned his mental fitness. I've had no reason to question his mental fitness," said Tillerson, whose office last year denied reports that he had referred to Trump as a "moron" after a national security meeting.

And, even in defending Trump, the former ExxonMobil chief executive admitted he has had to learn how to relay information to a president with a very different decision-making style.

"I have to learn how he takes information in, processes it and makes decisions," Tillerson told CNN. "I'm here to serve his presidency. So I've had to spend a lot of time understanding how to best communicate with him."

Tillerson insisted that the United States is in a stronger place internationally thanks to Trump's policies.

"He is not a typical president of the past, I think that's well recognised - that's also why the American people chose him," he said.

Tillerson's defence came as Washington devoured a new tell-all - Michael Wolff's "Fire and Fury: Inside the Trump White House" - that was rushed into bookstores after the White House failed in an attempt to suppress it.

The book quickly sold out in shops in the US capital, with some even lining up at midnight to get their hands on it and others circulating pirated copies. Trump has decried the instant best-seller as "phony" and "full of lies".

Journalist Wolff, no stranger to controversy, quotes several key Trump aides expressing doubt about Trump's ability to lead the world's largest economy and military hegemon.

"Let me put a marker in the sand here. One hundred percent of the people around him" question Trump's fitness for office, Wolff said in an interview on NBC's "Today" show.

"They all say he is like a child. And what they mean by that is he has a need for immediate gratification. It's all about him."

The 71-year-old Republican president, approaching the first anniversary of his inauguration, responded with fury.

"I authorized Zero access to White House (actually turned him down many times) for author of phony book! I never spoke to him for book. Full of lies, misrepresentations and sources that don't exist," Trump tweeted Thursday.

But Wolff countered: "I absolutely spoke to the president. Whether he realised it was an interview or not. I don't know, but it certainly was not off the record."

Despite dismissing the contents of the book, Trump has lashed out at his former aide Steve Bannon for being quoted as criticising the president’s family.

Trump has also been reported to be unimpressed with Tillerson’s work as secretary of state. Tillerson has made several statements that contradict Trump’s policies, including defending the Iran nuclear deal and refusing to take the side of Saudi Arabia against Qatar in the Gulf crisis.