US Defence Secretary James Mattis has said that he never spoke to the President about resigning from his position and was continuing with his job, after Donald Trump called him a "sort of a Democrat" and said that the Pentagon chief may be leaving his post.

Speaking to reporters en route to Vietnam, 68-year-old Mattis said he had never registered for any political party, having joined the military when he was 18 and adhered to its apolitical underpinnings as Republicans and Democrats alike came and went as Commander-in-Chief, the Washington Post reported on Monday. "I think he's sort of a Democrat, if you want to know the truth," Trump had said in an interview on CBS News's "60 Minutes" aired on Sunday.