"Make no mistake ― any attack on the United States or our allies will be defeated," the Pentagon chief said Saturday at a press conference in the South Korean capital, Seoul. "Any use of nuclear weapons by the North will be met with a massive military response, effective and overwhelming," presstv.com reported.
Tensions have been building on the Korean Peninsula following a series of nuclear and missile tests by Pyongyang as well as threats of war and personal insults traded between US President Donald Trump and North Korean leader Kim Jong-un.
Mattis stressed that diplomacy was still America’s ‘preferred course of action’ to resolve the standoff, but added, "Our diplomats are most effective when backed by credible military force.”
The defense secretary said North Korea needed to understand that its nuclear and missile programs would not strengthen its security.
“If it remains on its current path of ballistic missiles and atomic bombs, it will be counter-productive, in effect reducing its security,” Mattis said.
North Korea, which carried out its sixth nuclear test last month, said it wants a nuclear-tipped missile capable of reaching the United States. The country said it will continue to develop the programs as a deterrent against the US and its regional allies, including South Korea and Japan.
The CIA has estimated that Pyongyang could be only months away from developing the ability to strike the United States with nuclear weapons.
Mattis visited the Demilitarized Zone (DMZ) that separates the two Koreas on Friday and was briefed on the posture of North Korean artillery.
South Korea President Moon Jae-in said Friday that the US deployment has been effective in deterring North Korea which has stepped up its nuclear and ballistic missile tests.
South Korean Defense Minister Song Young-moo told his American counterpart that the US and South Korean missile deployments could not stop all of them. “Defending against this many LRAs (long-range artillery) is infeasible in my opinion,” Song said at the DMZ.
"We stand shoulder to shoulder with you and the Korean people in confronting the threats posed by the Kim Jong-un Government," Mattis said.
Trump, who has threatened to ‘totally destroy’ North Korea if necessary, leaves on his first trip to Asia next week, including a stop in South Korea to meet Moon.
Moon, after talks with Mattis on Friday, hailed the ‘aggressive deployment’ of US strategic weapons systems in the region.
Joint US military exercises with forces from South Korea and Japan have been a constant cause of tensions with North Korea. World leaders have called for restraint by all sides to the conflict.
The US and South Korea conducted a five-day naval drill, which began October 16, and involved the nuclear aircraft carrier Ronald Reagan, three nuclear submarines, Aegis destroyers and more than 40 other battleships and fighter aircraft ‘of all kinds.’
Mattis said Saturday that North Korea should ‘harbor no illusion’ about the fact that it is militarily ‘over-matched’ by the US and South Korea, home to 28,500 American troops.
Some Trump’s advisers say US military options are limited at best, saying Pyongyang could launch an artillery barrage on Seoul, which is only about 50 kilometers from the border.