U.S. Secretary of Defense James Mattis says that war with North Korea -- should tensions ever come to that -- would be "catastrophic."

"A conflict in North Korea, John, would be probably the worst kind of fighting in most people's lifetimes," Mattis told CBS News' "Face the Nation" host John Dickerson in his first official interview as defense secretary.

The North Korean regime has hundreds of artillery cannons and rocket launchers within range of one of the most densely populated cities on Earth -- Seoul, the capital of South Korea, Mattis said.

North Korea is a threat to the surrounding region, including Japan, China and Russia, he said.

"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," Mattis said.

The defense secretary said North Korea isn't only a threat abroad, calling it 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 intercont- intercontinental ballistic missile with a nuclear weapon on it to say that now it's manifested completely," Mattis said.

North Korea has attempted seven missile tests in 2017, including two recent tests in one week alone, keeping the world on edge.

"We always assume that with a testing program they get better with each test," Mattis said.

Mattis declined to say when North Korea could reach a red-line point of no return.

"We consider it a direct threat even today, the North Korean threat," Mattis said. "As far as that specific threat, 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."

Preparing for North Korea's growing threat, the Pentagon will attempt to shoot down an intercontinental-range missile for the first time in a test this week, with the goal of more closely simulating a North Korean ICBM aimed at the U.S.

The American interceptor has succeeded in nine of 17 attempts since 1999. The most recent test in June 2014 was a success, but that was only after three failures. North Korean leader Kim Jong-un has vowed to possess a missile capable of reaching the U.S., and though he hasn't yet tested such a missile, Pentagon officials are on their toes.

In the 2018 budget the White House offered to Congress earlier this week, the Pentagon proposed spending $7.9 billion on missile defense.