Secretary of Defense Jim Mattis on Tuesday pushed back hard against a new book containing shocking revelations about how President Donald Trump has allegedly operated and how his most senior aides reportedly have little respect for him — allegations that have immediately raised questions about how long some of his advisers would continue to serve.

Among the revelations in veteran journalist Bob Woodward’s new book, “Fear: Trump in the White House," Trump allegedly called for the assassination of Syrian president Bashar Assad after the 2017 chemical strike.

“Trump called Mattis and said he wanted to assassinate the dictator,” according to a passage from the book, which will be released Sept. 11.

“Let’s f**king kill him! Let’s go in. Let’s kill the f**king lot of them,” Trump said, according to Woodward.

Mattis told the president that he would get right on it.

But after hanging up the phone, he told a senior aide: “We’re not going to do any of that. We’re going to be much more measured.”

The national security team then developed options for the more conventional airstrike that Trump ultimately ordered.

Mattis, who was traveling to India when the news stories broke, issued a statement challenging that passage and others, including one where he allegedly said Trump "acted like — and had the understanding of — ‘a fifth- or sixth-grader,’ ” when Mattis was trying to explain to him the importance of keeping U.S. troops on the Korean Peninsula.

Trump has repeatedly canceled U.S.-Korea joint exercises, saying they cost too much and provoke North Korea.

“The contemptuous words about the President attributed to me in Woodward’s book were never uttered by me or in my presence,” Mattis said in a statement. “While I generally enjoy reading fiction, this is a uniquely Washington brand of literature, and his anonymous sources do not lend credibility.”

Mattis went on to cite his work to date with Trump to improve military readiness, strike the Islamic State and get NATO allies to contribute more to defense.

Mattis is not the only official in the book who allegedly attacked Trump’s intellect or decision-making. Retired Marine Corps Gen. John Kelly, the White House chief of staff, is quoted as saying the president is an idiot, and that it’s the worst job he’s ever had.

Kelly issued a similar denial through the White House.

“The idea I ever called the President an idiot is not true," Kelly said in a statement.

He called the book passage “another pathetic attempt to smear people close to President Trump and distract from the administration’s many successes.”

Similar alleged statements by former Secretary of State Rex Tillerson, who reportedly called Trump a “moron,” were cited as one of the reasons Tillerson ultimately left the position after just a year on the job, leading to questions about Mattis' and Kelly’s own tenures.

It’s been speculated that Mattis, who has been out of the loop on several key White House decisions, may also be leaving the administration before Trump’s first term is complete. Aides close to Mattis have previously said that they expect he will stay for the whole term if asked, due to some of the internal Pentagon readiness issues he is closely focused on and would like to see through to fruition.

“In serving in this administration, the idea that I would show contempt for the elected Commander-in-Chief, President Trump, or tolerate disrespect to the office of the President from within our Department of Defense, is a product of someone’s rich imagination," Mattis said.