"A conflict in North Korea would be probably the worst kind of fighting in most people's lifetimes. Why do I say this? 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.

"We are working with the international community to deal with this issue. 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 said. "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."

Mattis also said the North makes progress on its missile program with each test regardless of if it's successful or not. He also said the North is a threat not only to the region but also "a direct threat to the United States."

"They have been very clear in their rhetoric we don't have to wait until they have an intercontinental ballistic missile with a nuclear weapon on it to say that now it's manifested completely," he said.

Mattis declined to talk about the "red line" about the North.

"The president needs political maneuver room on this issue. We do not draw red lines unless we intend to carry them out. We've made very clear that we're willing to work with China, and we believe China has tried to be helpful in this regard," he said.

Asked about when the North would get to the point of no return, Mattis only said, "I don't want to put a timeline on it. At this time, what we know, I'd prefer to keep silent about because we may actually know some things the North Koreans don't even know." (Yonhap)