Secretary of State Rex Tillerson Rex Wayne TillersonGary Cohn: 'I haven't made up my mind' on vote for president in November Kushner says 'Alice in Wonderland' describes Trump presidency: Woodward book Conspicuous by their absence from the Republican Convention MORE said Friday that he has never questioned President Trump Donald John TrumpHR McMaster says president's policy to withdraw troops from Afghanistan is 'unwise' Cast of 'Parks and Rec' reunite for virtual town hall to address Wisconsin voters Biden says Trump should step down over coronavirus response MORE's mental fitness amid a public discussion about the topic following the release of a bombshell new book.

"I've never questioned his mental fitness," Tillerson told CNN's Elise Labott in an interview. "I have no reason to question his mental fitness."

Labott pointed to questions raised about Trump's mental fitness following author Michael Wolff's explosive book "Fire and Fury: Inside the Trump White House," which went on sale Friday.

Wolff wrote that even Trump's own aides have shared concerns about Trump's mental state, writing, “My indelible impression of talking to them and observing them through much of the first year of his presidency, is that they all — 100 percent — came to believe he was incapable of functioning in his job."

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The White House has slammed questions about the president's mental fitness for office following the release of excerpts from the book earlier this week.

“It’s disgraceful and laughable,” White House press secretary Sarah Huckabee Sanders told reporters on Friday. “If he was unfit, he probably wouldn’t be sitting there.”

The relationship between Tillerson and the president has been the focus of intense media interest for months.

Reports surfaced over the past year detailing tension between the two men, including one that said Tillerson referred to Trump as a "moron," which the State Department later denied.

Trump also appeared to refute Tillerson in October on the issue of North Korea, saying his top diplomat was "wasting his time" on talks with Pyongyang.

Tillerson said on Friday his relationship with Trump was "developing," and that he plans on staying in his diplomatic post for the whole year.

"I think we're going to have a very productive 2018," Tillerson said. "The State Department gets stronger every day, understand what we're trying to do. And I look forward to having a very, very successful 2018."