White House press secretary Sarah Sanders fired back on Thursday at pundits who have questioned President Trump's mental fitness for office in recent days, telling reporters at the White House that Trump would not have beaten a well-qualified field of GOP primary opponents if he wasn't mentally healthy.

"It's disgraceful and laughable," Sanders said of criticism aimed at Trump's mental state.

"If he was unfit, he probably wouldn't be sitting there," she added.

Questions about Trump's fitness have intensified this week amid the publication of excerpts from a book, "Fire and Fury: Inside the Trump White House," that gives a close look at the early months of the administration.

The press secretary said Thursday the book is "full of false and fake information." Trump has threatened to take the book's author, Michael Wolff, and his publisher to court in an effort to stop publication of the book, which is slated for Tuesday.

"This is an incredibly strong and good leader, that's why we've had such a successful 2017," Sanders said.