U.S. Secretary of Defense James Mattis on Friday said that military solutions to North Korea’s nuclear and missile programs would be “tragic on an unbelievable scale” at a Pentagon press conference.

But the former Marine Corps general did not rule out military action, instead saying that the U.S. would prefer diplomatic solutions.

“We’re going to continue to work the issue … If this goes to a military solution, it is going to be tragic on an unbelievable scale. So our effort is to work with the [UN], work with China, work with Japan, work with South Korea to try to