U.S. Secretary of Defense James Mattis on Saturday said the nuclear threat posed by North Korea is “accelerating” and warned Pyongyang of a “massive military response” to any nuclear weapons use. Mattis, speaking at a joint press conference with his South Korean counterpart Song Young-Moovisit during a visit to Seoul, said collaboration between the U.S. and South Korea has taken on a “new urgency” in recent days. “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. While diplomacy is still the “preferred course of action,” he warned of an “effective and overwhelming” U.S. military response should dialogue not be enough. “Make no mistake—any attack on the United States or our allies will be defeated,” he said. “I cannot imagine a condition under which the United States will accept North Korea as a nuclear power,” he said. Mattis’ comments come after Pyongyang's Foreign Minister Ri Yong-Ho said last month that North Korea could soon test a nuclear bomb over the Pacific.