“Why would I do that?” Trump told reporters on Monday when asked whether he’d replace Mattis. “Is that the new rumour? No.”

One of Trump’s first cabinet picks after the 2016 election, Mattis has long been seen as a force for stability in foreign policy under a president who prides himself on his unpredictability.

The Pentagon secretary found himself on the defensive, however, after he was portrayed as critical of Trump in Bob Woodward’s book “Fear,” published in early September.

Mattis pushed back early in 2018 when Trump questioned the massive US military presence on the Korean peninsula, asking why the US was spending so much, Woodward reported.

“We’re doing this in order to prevent World War III,” Mattis said, according to the book.

He went on to tell associates that the president had the understanding of a “fifth or sixth-grader,” according to Woodward.

It also quoted the secretary as saying that defence chiefs “don’t always get to choose the president they work for.”