US Defence Secretary Jim Mattis - during a visit to South Korea - is accusing the North of illegal and unnecessary missile and nuclear programs, and pledging to repel any strike.
Mattis said North Korea engages in in what he calls "outlaw" behaviour and he says the US will never accept a nuclear North.
Regardless of what the North might try, it is overmatched by the firepower and cohesiveness of the decades-old US-South Korean alliance, the US defence boss said.
Mattis says nukes undermine North Korea
US Defence Secretary Jim Mattis said he could not imagine United States ever accepting a nuclear North Korea, and warn that its rapidly advancing nuclear and missile programs would undermine, not strengthen, its security.
Mattis has been at pains during his week-long trip to Asia to stress that diplomacy is America's preferred course, a message he returned to after top-level military talks in Seoul on Saturday and at the tense border area with
North Korea on Friday.
Still, he warned Pyongyang that its military was no match for the US-South Korean alliance, and that diplomacy was most effective "when backed by credible military force."
"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 both effective and overwhelming," Mattis said.
Mattis's South Korean counterpart, Defence Minister Song Young-moo, dismissed the idea of deploying tactical nuclear weapons to the peninsula as a response to the North's advances.
But the US-Korean alliance had the ability to respond, even in the event of a nuclear attack from the North, Song added.
Tension between North Korea and the United States has been building after a series of nuclear and missile tests by Pyongyang and bellicose verbal exchanges between North Korean leader Kim Jong Un and President Donald Trump.
The CIA has said North Korea could be only months away from developing the ability to hit the United States with nuclear weapons, a scenario Trump has vowed to prevent.
