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 on Saturday warned that the threat of North Korea launching a nuclear missile attack has "accelerated."

“North Korea has accelerated the threat that it poses to its neighbors and the world through its illegal and unnecessary missile and nuclear weapons programs,” Mattis said during a visit to South Korea, alongside Song Young-moo, the country's defense minister, in comments reported by The Associated Press.

Mattis went on to accuse North Korea of "outlaw" behavior and vowed the U.S. would never accept a nuclear North Korea, despite the country's persistence in developing nuclear weapons.

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“I cannot imagine a condition under which the United States would accept North Korea as a nuclear power,” he said.

“Make no mistake — any attack on the United States or our allies will be defeated, and any use of nuclear weapons by the North will be met with a massive military response that is effective and overwhelming," he continued.

Mattis's comments come less than a week before President Trump will embark on his first trip to Asia as president.

Trump has ratcheted up rhetoric toward North Korea in light of the country's recent intercontinental ballistic missile tests.

The president vowed on the floor of the United Nations last month to "totally destroy" North Korea if it attacked the U.S. or its allies, prompting Pyongyang to threaten to drop a hydrogen bomb over the Pacific Ocean.

Mattis visited the Demilitarized Zone between North and South Korea on Friday. It remains unclear whether Trump will visit the same area.

Trump appeared to raise the possibility of a potential presidential visit to the zone at the White House on Wednesday.

“Well, I'd rather not say, but you'll be surprised,” he said.