Story highlights The Pentagon chief made his remarks one day after visiting the demilitarized zone

Mattis' trip comes as Trump prepares to travel to the region next week

(CNN) Secretary of Defense James Mattis said Saturday that the increasing nuclear threat from North Korea has brought "a new urgency" to military and diplomatic collaboration with South Korea.

"The threat from North Korea has grown markedly even since my trip here earlier this year," Mattis said while appearing alongside his South Korean counterpart, Song Young-moo, at a press conference in Seoul.

"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," he added, emphasizing that the US "does not accept a nuclear North Korea."

The Pentagon chief made his remarks one day after visiting the demilitarized zone that separates North and South Korea and meeting with South Korean President Moon Jae-in at the country's presidential palace.

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