US Defense Secretary Jim Mattis said Saturday the threat of nuclear missile attack by North Korea is accelerating, warning Pyongyang of a "massive military response" to any use of nuclear weapons. In remarks in Seoul with South Korean Defense Minister Song Young-moo at his side, Mattis accused the North of illegal and unnecessary missile and nuclear programs — and vowed to defeat any attack. Mattis said North Korea engages in "outlaw" behavior and that the US will never accept a nuclear North. He added that regardless of what the North might try, it is overmatched by the firepower and cohesiveness of the decades-old US-South Korean alliance. "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 said, adding that US-South Korean military and diplomatic collaboration thus has taken on "a new urgency." "I cannot imagine a condition under which the United States would accept North Korea as a nuclear power," he said. Mattis, on a trip to Seoul for annual defense talks, maintained that diplomacy remained a "preferred course of action" but stressed, "our diplomats are most effective when backed by credible military force". "With that said," he added, "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." Mattis trip comes ahead of Trumps first presidential visit to South Korea next month as part of his Asia tour which also includes Japan, China, Vietnam and the Philippines.  Trump entered office declaring his commitment to solving the North Korea problem, asserting that he would succeed where his predecessors had failed. His administration has sought to increase pressure on Pyongyang through UN Security Council sanctions and other diplomatic efforts, but the North hasnt budged from its goal of building a full-fledged nuclear arsenal, including missiles capable of striking the US mainland.