No reason to question Trump's mental fitness: Tillerson

US Secretary of State Rex Tillerson has never questioned President Donald Trump's mental fitness and intends to remain at his post for the entire year.

The top US diplomat made the remarks in an exclusive interview with CNN. The American news agency published a report about the interview on Friday.

Tillerson was responding to a question about Michael Wolff's explosive book, "Fire and Fury," a detailed look at the first year of the Trump presidency that is raising new questions about the President's temperament.

"I have never questioned his mental fitness. I have no reason to question his mental fitness," Tillerson said

During the interview, he addressed the threat of North Korea's nuclear programmne, tensions with Russia and Iran, and his famously tense relationship with the commander in chief.

Tillerson had reportedly dismissed Trump in the past as a "moron," while the President publicly hinted that he would win an IQ contest against his top diplomat.

Tillerson said that his relationship with Trump is evolving, and one that he's been learning to navigate during his first year on the job.

While Trump repeatedly seemed to undermine Tillerson's diplomatic efforts in tweets, and White House sources regularly leaked stories damaging to the former ExxonMobil CEO, Tillerson said he "learned over the past year better how to deal with the president."

Tillerson also pushed back against characterisations in the Wolff book that Trump lacks the attention span and focus to get through meetings with foreign leaders, sometimes leaving them early.

"I have never seen the president leave a meeting with a foreign leader," Tillerson said.

"Among all the Cabinet secretaries, I probably have spent more time with the President than perhaps Secretary (of) Defense (James) Mattis, who spends a lot of time with him as well," Tillerson said.