SEOUL • US Secretary of Defence James Mattis yesterday warned North Korea of a "massive military response" to any use of nuclear weapons as tensions remain sky-high ahead of President Donald Trump's visit to South Korea.

Mr Mattis, in Seoul for annual defence talks, maintained that diplomacy remained a "preferred course of action" but stressed that "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," he said at a 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."

He did not specify the threshold of nuclear weapon activity that would trigger a military response.

North Korea's Foreign Minister Ri Yong Ho had said on the sidelines of the United Nations General Assembly last month that his country could test a nuclear bomb over the Pacific.

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

Mr Mattis' trip comes ahead of Mr Trump's first presidential visit to South Korea next month as part of his Asia tour which includes Japan, China, Vietnam and the Philippines. All eyes will be on his message to North Korea and its leader Kim Jong Un.

Mr Trump is expected to deliver a speech at the South's Parliament and to visit a US military base during his Nov 7-8 trip to Seoul.

His recent remark that "only one thing will work" with the North fuelled concerns of a potential conflict on the divided peninsula where the 1950-53 Korean War had left millions dead.

AGENCE FRANCE-PRESSE