WASHINGTON - US Secretary of Defense James Mattis is one of the longest serving members of US President Donald Trump's cabinet, but a remark made by Trump on Sunday has put Mattis's position into doubt.

"I think he's sort of a Democrat, if you want to know the truth," Trump said in an interview that aired Sunday on the TV show 60 Minutes, adding: "But General Mattis is a good guy. We get along very well. He may leave. I mean, at some point, everybody leaves. Everybody. People leave. That's Washington."

Mattis, seen as one of the steadiest but also more independent members of Trump's cabinet, has served as a low-profile counterweight to the president in his often abrasive treatment of US allies.

Trump said he and Mattis enjoyed a "very good relationship", but the remark nevertheless raised questions about Mattis's future as the US defense chief.

Pentagon spokesman Robert Manning said in a statement in response to Trump's remark on Sunday that "Secretary Mattis is laser-focused on doing his job－ensuring the US military remains the most lethal force on the planet".

Trump's comments came as he eyes another cabinet shakeup.

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.

Rumors about a potential exit for Mattis, often labeled a grown-up figure in the Trump administration, have been circulating for weeks.

Put into spotlight

Mattis was put into the spotlight by a book published earlier that shed light on the inner workings of the Trump administration.

Inside the book, written by veteran journalist Bob Woodward, Mattis was said to have brushed off a directive from Trump to assassinate Syrian President Bashar al-Assad.

Although Mattis denied remarks attributed to him in the book, the impression that Mattis and Trump shared different views on certain issues lingered.

Mattis, 68, was a four-star general in the US Marine Corps until 2013. Trump nominated him to head the Pentagon after winning the election. Mattis was confirmed by the Senate in a 98-1 vote.

Asked about the reports speculating about his departure, Mattis said on Sept 18: "It's like most of those kinds of things in this town.

"Somebody cooks up a headline. They then call to a normally chatty class of people. They find a couple of other things to put in. They add the rumors... Next thing you know, you've got a story," he said.

Xinhua/Reuters/AFP