SEOUL: US Secretary of Defense Jim Mattis on Saturday warned North Korea of a "massive military response" to any use of nuclear weapons as tensions remain sky-high ahead of Donald Trump's visit to South Korea, reports AFP.

Pyongyang in recent months has sparked global alarm by conducting a sixth nuclear test and test-launching missiles capable of reaching the US mainland, while Trump and the North's young ruler Kim Jong-Un have traded threats of war and personal insults.

Mattis, on a trip to Seoul for annual defence talks, maintained that diplomacy remained a "preferred course of action" but stressed, "our diplomats are most effective when backed by credible military force".

"Make no mistake -- any attack on the United States or our allies will be defeated," Mattis said at a joint press conference with his South Korean counterpart Song Young-Moo.

"Any use of nuclear weapons by the North will be met with a massive military response, effective and overwhelming," Mattis said, adding Washington "does not accept a nuclear North Korea."

"I cannot imagine a condition under which the United States will accept North Korea as a nuclear power," he said.

Mattis did not specify the threshold of nuclear weapon activity that would trigger a military response. Pyongyang's Foreign Minister Ri Yong-Ho said on the sidelines of the UN General Assembly last month that his country could test a nuclear bomb over the Pacific.

But Mattis said Pyongyang should "harbour no illusion", saying the isolated state is militarily "overmatched" by the US and South Korea -- a key ally of Washington that hosts 28,500 US troops.

Mattis' trip comes ahead of Trump's first presidential visit to South Korea next month as part of his Asia tour which also includes Japan, China, Vietnam and the Philippines.

All eyes will be on Trump's message to the North and Kim.