Washington: US President Donald Trump has said many things about many people, including his own lieutenants, but Sunday marked the first time he's said anything negative about his Defence Secretary James Mattis.

"It could be that he is" planning to depart, Trump said. "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. Everybody. People leave. That's Washington."

Trump said he was unsure whether Mattis was planning to step down from his post, but told CBS' 60 Minutes in a pre-taped interview that the retired general might.