THE threat of nuclear missile attack by North Korea is accelerating, US Defence Secretary Jim Mattis claimed during a visit to South Korea.

In remarks in Seoul with South Korean Defence Minister Song Young-moo at his side, Mattis accused North Korea of illegal and unnecessary missile and nuclear programmes – and vowed to defeat any attack. Adding the US will never accept North Korea becoming a nuclear state.

"North Korea has accelerated the threat that it poses to its neighbours and the world through its illegal and unnecessary missile and nuclear weapons programmes," he said, adding that US-South Korean military and diplomatic collaboration has taken on "a new urgency".

As he emphasised throughout his week-long Asia trip diplomacy remains the preferred way to deal with the North.

"With that said," he added, "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 effective and overwhelming."

Mattis's comments in Seoul did not go beyond his recent statements of concern about North Korea, although he appeared to inject a stronger note about the urgency of resolving the crisis.

While he accused the North of "outlaw" behaviour, he did not mention that President Donald Trump has ratcheted up his own rhetoric. In August Trump warned the North not to make any more threats against the United States and said that if it did, it would be met with "fire and fury like the world has never seen".

North Korea says it needs nuclear weapons to counter what it believes is a US effort to strangle its economy and overthrow its government.