Defense Secretary James Mattis James Norman MattisBiden courts veterans amid fallout from Trump military controversies Trump says he wanted to take out Syria's Assad but Mattis opposed it Gary Cohn: 'I haven't made up my mind' on vote for president in November MORE warned on Sunday that a war with North Korea would be "catastrophic," and would place United States allies in the region at extreme risk.

"The North Korean regime has hundreds of artillery cannons and rocket launchers within range of one of the most densely populated cities on earth, which is the capital of South Korea," Mattis said on CBS's "Face the Nation."

"This regime is a threat to the region, to Japan, to South Korea. And in the event of war, they would bring danger to China and to Russia as well," he added. "But the bottom line is it would be a catastrophic war if this turns into a combat if we're not able to resolve this situation through diplomatic means."

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Tensions rose between Washington and Pyongyang last month over the North's rapidly advancing nuclear and missile programs, prompting the U.S. to move a Navy strike group into the west Pacific near the Korean Peninsula. North Korea denounced that maneuver as an act of aggression and threatened a strike on the U.S. if provoked.

The exchange put other countries in the region on high alert, particularly South Korea and Japan, where the U.S. has tens of thousands of troops stationed.

President Trump Donald John TrumpObama calls on Senate not to fill Ginsburg's vacancy until after election Planned Parenthood: 'The fate of our rights' depends on Ginsburg replacement Progressive group to spend M in ad campaign on Supreme Court vacancy MORE told Reuters in an interview with Reuters last month that a "major, major conflict" between the U.S. and North Korea was possible, and that while he preferred a diplomatic solution, such a path would be "very difficult."