US Secretary of Defense James Mattis on Thursday joked that National Security Adviser John Bolton was "the devil incarnate."

"I heard you're actually the devil incarnate and I wanted to meet you," Mattis said upon Bolton's arrival at the Pentagon.

Bolton responded by laughing, and the two men walked into the Pentagon.

Earlier this week, Mattis told reporters that his relationship with Bolton would be a "partnership," and the two men were "going to go forward."

Also on Thursday, Mattis said of Bolton, "Last time I checked, he’s an American and I can work with an American. Okay? I'm not the least bit concerned with that sort of thing."