"The US does not accept a nuclear North Korea… Make no mistake, any attack on the US or our allies will be defeated. Any use of nuclear weapons by the North will be met with a massive military response, effective and overwhelming, “he said during a speech in Seoul, Sputnik reported.
Mattis said that by sticking to its nuclear and missile program the North was in effect "reducing its security," after Pyongyang justified its research of nuclear weapons by a threat from the United States.
The Pentagon chief stressed that dialogue remained Washington's preferred method of settling the dispute with Pyongyang but added the threat of a nuclear strike from the North was accelerating.
During his fourth trip to Asia in his current post, Mattis went to South Korea on Friday to co-chair the 49th annual Security Consultative Meeting. He also visited the demilitarized buffer zone between the two Koreas with South’s Defense Minister Song Young-moo.
The United States and South Korea, Mattis said, remain steadfast allies in the face of a threat from the North, which is pushing ahead with efforts to develop nuclear-tipped missiles capable of reaching the continental United States.