Oct. 28 (UPI) -- Defense Secretary James Mattis said Saturday in Seoul, South Korea, that the threat of North Korea launching a nuclear missile attack has "accelerated" but warned the regime that "outlaw" behavior won't be tolerated.
The U.S. leader, appearing alongside his South Korean counterpart, Song Young-moo, at a news conference said "as you know, our military and diplomatic collaboration has taken on a new urgency."
On Friday, he and Song visited the Demilitarized Zone, which is the official buffer between the two Koreas.
"North Korea has accelerated the threat that it poses to its neighbors in the world through its illegal and unnecessary missile and nuclear weapons programs," Mattis said. "It engages in outlaw behavior, threatening behavior, condemned by unanimous ... Security Council resolutions."
Mattis said "diplomacy remains our preferred course of action. As I have repeatedly emphasized our diplomats are most effective when backed by credible military force in this sort of situation."
But he noted force will be met with force.
"Make no mistake any attack on the United States or our allies will be defeated," he said of Kim Jong Un's threats to use his nuclear weapons. "Any use of nuclear weapons by the North will be met with a massive military response, effective and overwhelming."
Mattis said the United States has "taken defensive steps as an alliance." That includes deploying the THAAD anti-missile system.
"Millions of [South Korean] citizens and our combined forces are now better protected by this fully defensive system," he said.
Song said that he and Mattis agreed to further cooperate on strengthening Seoul's defense capabilities, including lifting warhead payload limits on South Korean conventional missiles and supporting the country's acquisition of "most advanced military assets."
Mattis and Song have rejected some South Korean politicians' calls for the United States to bring back tactical nuclear weapons withdrawn from the Korean Peninsula in the 1990s.
"When considering national interest, it's much better not to deploy them," Song said.
Mattis said current U.S. strategic assets are already providing nuclear deterrence.
"In terms of tactical nukes on the peninsula, I want to remind everyone that unlike the Kim [Jong Un] regime in North Korea, our combined effort is to denuclearize the peninsula," Mattis said. "That's also the effort of the United Nations and the People's Republic of China and I could go on. This is a global concern right now. And I would only add to that, we have a global strategic capability that is a deterrent, and I'll just tell you that this subject has not been broached to me by the Republic of Korea government."
The U.S. defense secretary said "we stand shoulder-to-shoulder with you and the Korean people in confronting the [North Korea's] threat."
Matis, a retired United States Marine Corps general, made his first South Korea visit in the early 1970s.
"I will tell you in over 40 years of working with allied nations, the collaboration level between the ROK and the United States are a model, perhaps the best example I've ever seen of close collaboration of trust, and affiliation and shared appreciation of dangers," he added.
Mattis and Song co-chaired the 49th Annual Security Consultative Meeting, an annual security dialogue between defense ministers of the two allies.
"We reviewed alliance progress over the last year, and set the agenda for coming years on a number of important issues," Mattis said. "Further, we ensured that our actions whether acquisition of inter-operational capabilities or the transfer of wartime operational control to ensure that this alliance grows stronger, not weaker, and our militaries are always ready to protect those threatened by the Kim regime."
"President Trump has made clear that America's commitments to defending our allies and to upholding our extended deterrence guarantee is 'ironclad,'" the secretary said.