Secretary of Defence says US 'not rushing to war' with North Korea

In recent months the North has staged its sixth nuclear test and fired a flurry of missiles, sparking a fiery war of words between US President Donald Trump and North Korean leader Kim Jong-Un.

But Mattis, who arrived in Thailand Thursday following a meeting with his ASEAN counterparts in the Philippines, said Washington was "not rushing to war" and looking for a diplomatic resolution.

"Do we have military options in defence if we're attacked, our allies are attacked? Of course we do," Mattis said. "But everyone is out for a peaceful resolution."

Following talks with his South Korean and Japanese counterparts on the sidelines of the security forum in Manila, the ministers agreed Pyongyang's weapons programme posed an "unprecedented and grave threat" and vowed to step up diplomatic pressure against the regime.

"And that's really what -- what it is all about, to keep DPRK efforts firmly in the diplomatic lane for resolution," Mattis said, using the acronym for the North's official name.

Mattis is on an Asian tour which will see him arriving in Seoul on Friday for annual defence talks, ahead of a visit to South Korea by Donald Trump next month.

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

The US president's recent remark that "only one thing will work" with North Korea fuelled concerns of a potential conflict.

But even some Trump advisers say US military options are limited when Pyongyang could launch an artillery barrage on the South Korean capital Seoul -- only around 50 kilometres from the heavily-fortified border and home to 10 million people.