WASHINGTON (AP) -- President Donald Trump says he has a "very good relationship" with Defense Secretary Jim Mattis even though he thinks the Pentagon chief is "sort of a Democrat."

Asked during a "60 Minutes" interview airing Sunday whether Mattis will step down, Trump says Mattis hasn't told him that.

But Trump says: "It could be that he is. I think he's sort of a Democrat, if you want to know the truth. But Gen. Mattis is a good guy. We get along very well. He may leave. I mean, at some point, everybody leaves."

Last month, Mattis dismissed news reports that tensions with Trump could soon point to his exit. One area of tension has reportedly been on the importance of NATO, the alliance of Western nations that was formed after World War II to help keep the peace.

Trump disputed reports that Mattis had said to him, "The reason for NATO and the reason for all these alliances is to prevent World War III."

Trump told "60 Minutes": "I think I know more about it than he does. And I know more about it from the standpoint of fairness, that I can tell you."

Trump has said before that he wants NATO countries to contribute more financially to support the alliance.

"We shouldn't be paying almost the entire cost of NATO to protect Europe. And then on top of that, they take advantage of us on trade."