WASHINGTON - United States Secretary of Defence James Mattis is departing on Monday (Oct 15) for a trip to Vietnam and Singapore, according to a press release issued by the US Department of Defence.

He will visit Vietnam and also travel to Singapore, where he will participate in the Asean Defence Ministers' Meeting. The meeting is scheduled for Thursday to Saturday.

Mr Mattis will meet key leaders to reaffirm defence relationships and conduct bilateral and trilateral meetings with senior officials, said the press release issued on Oct 12.

Over the weekend, fresh comments made by US President Donald Trump have led to renewed questions over the fate of the retired Marine four-star general.

Mr Trump said in an interview airing on Sunday that Mr Mattis "could be" leaving, referring to him as "sort of a Democrat".

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

The New York Times reported last Saturday that the relationship between the two men may have "soured" to the point of no return.

In an interview to be aired on Sunday on CBS's 60 Minutes, Mr Trump was asked whether he wanted Mr Mattis to leave. According to an excerpt released by CBS, Mr Trump said: "It could be that he is. I think he's sort of a Democrat, if you want to know the truth."

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

According to news reports, concerns that Mr Mattis's days as defence chief are numbered have mounted since a book titled Fear by journalist Bob Woodward said the retired general had questioned Mr Trump's judgment, likening his understanding to that of a 10- or 11-year-old child.

"Of course, I don't think about leaving," Mr Mattis told Pentagon reporters last month. "I love it here."