Seoul: Visiting US Secretary of Defence James Mattis warned on Saturday that North Korea will be defeated with a "massive military response" if it attacks his country or its allies.
He said he "can't imagine" accepting North Korea as a nuclear power, reports Yonhap News Agency.
"Make no mistake, any attack on the United States or our allies, will be defeated," he said at a joint press conference with his South Korean counterpart Song Young-moo after their talks here.
"Any use of nuclear weapons will be met with a massive military response."
His remarks came after the conclusion of the 49th annual Security Consultative Meeting (SCM) between the US and South Korea, held at the Defence Ministry.
He pointed out the allies' coordination has taken on new urgency as North Korea has accelerated its threats with "illegal" and "unnecessary" nuclear and missile programmes.
The North's military is overmatched by the allies' combined defence posture, he stressed.
"If it remains on its current path of ballistic missiles and atomic bombs, it will be counterproductive, in effect, reducing its own security," the Pentagon chief said.
Upon his arrival in South Korea on Friday, Mattis said that the American government did not want a war with North Korea, but rather initiate a process to achieve a complete denuclearisation of the Korean peninsula.
Mattis' statement came during his visit to a border guard post and the Joint Security Area, which is part of the Demilitarized Zone (DMZ) that divides the two Koreas.
He urged the Kim Jong-un regime to halt provocations that threaten others "with catastrophe".
Mattis' trip to the region comes ahead of US President Donald Trump's visit to Asia next week.
However, Trump will most likely forgo a visit to the heavily fortified border, according to the White House.