WASHINGTON (BLOOMBERG) - President Donald Trump hinted that United States Defence Secretary Jim Mattis might be planning to quit his administration, and described the former US Marine Corps general as "sort of a Democrat".

In an excerpt from a 60 Minutes interview that will air in full Sunday (Oct 14) US time on CBS, Mr Trump talked about the level of turnover in his administration after almost two years, and suggested there could be more to come.

"There are some people that I'm not happy with. I have some people that I'm not thrilled with. And I have other people that I'm beyond thrilled with," the president said, according to a transcript provided by CBS.

Mr Mattis "hasn't told me" that he plans to leave, Mr Trump said, and didn't directly answer the question whether he'd like the Pentagon chief to depart.

"I had lunch with him two days ago. I have a very good relationship with him. It could be that he is. I think he's sort of a Democrat, if you want to know the truth," Mr Trump said. "He may leave. I mean, at some point, everybody leaves. Everybody. People leave. That's Washington."

Mr Mattis was one of Mr Trump's first Cabinet picks after the 2016 election. He was long seen as a force for stability in foreign policy in an administration that has had to manage crises from North Korea to Syria under a president who prides himself on his unpredictability.

But the Pentagon chief has been on the defensive after excerpts from author Bob Woodward's book Fear painted the publicly taciturn military man as being critical of Mr Trump in private.

Mr Trump's comments come ahead of a visit to Vietnam and Singapore this week by Mr Mattis. He departs Monday (Oct 15) to visit Vietnam. Later, he travels to Singapore where he will participate in the Association of South-east Asian Nations Defence Ministers' Meeting.