Speaking to reporters as he flew from Washington to Ho Chi Minh City at the start of a week-long Asia trip, Mattis said he had spoken directly to Trump by phone late Monday morning, Washington time.

"He said, 'I'm 100 percent with you'," Mattis said.

His remark comes after US broadcaster CBS on Sunday aired an interview with Trump, in which he suggested Mattis may be headed out the Pentagon door.

"It could be that he is. I think he's sort of a Democrat, if you want to know the truth," Trump said.

"But General Mattis is a good guy. We get along very well. He may leave. I mean, at some point, everybody leaves."

Earlier on in the 20-hour flight to Vietnam -- before he'd spoken to Trump -- Mattis was asked what he'd made of Trump's comments.

"Nothing at all," Mattis said.

"I'm on his team. We have never talked about me leaving. And as you can see right here, we are on our way, we just continue doing our job... no problem."

He added that he had never registered with any political party, and that those in the military are "proudly apolitical".

- Cabinet reshuffle -

Mattis's fate has been the subject of intense speculation for months.

It reached fever pitch in September, when veteran journalist Bob Woodward published a book giving an inside glimpse into a chaotic White House and said Mattis had questioned Trump's judgment, likening his understanding to that of a 10- or 11-year-old child.

Mattis, a buttoned-up former Marine Corps general, is loathe to discuss politics or his relationship with Trump, and the president is widely reported to bristle at unfavourable comparisons to Mattis, who has broad bipartisan support.

The Woodward book details a number of times Mattis or Trump officials are said to have slow-walked orders from the president, such as in the aftermath of Trump's tweets last year that said all transgender personnel would be banned from the military.

Trump is looking to reshape his cabinet after the mid-term elections, which will determine whether the Democrats can regain some degree of power after a total and humiliating defeat in 2016.

The president has already cast off many of his original top-level picks, including former secretary of state Rex Tillerson and Lieutenant General HR McMaster, who had been national security advisor.

Last week, his ambassador to the United Nations, Nikki Haley, announced her resignation, effective at the end of the year.

She gave no reason for the surprise move, but observers have noted that the ascension of John Bolton as national security adviser and Mike Pompeo as secretary of state have shifted the power dynamics on Trump's national security team.

The many ways the scholarly Mattis's worldview contrasts with that of his boss is an evergreen subject of stories in Washington, a fact that rankles the Pentagon chief, who says he despises being portrayed as an "adult" compared to Trump.

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