US Defense Secretary Jim Mattis warned Thursday that war with North Korea would be "catastrophic" and said diplomatic efforts were yielding results.

"The American effort is diplomatically led, it has diplomatic traction, it is gaining diplomatic results and I want to stay right there right now," Mattis said at an event in California.

"The tragedy of war is well-enough known it doesn’t need another characterization beyond the fact that it would be catastrophic," he added.