Recalling his phone conversation with South Korean Defense Minister Song Young-moo and the phone call between the two countries' leaders earlier this week, he added, "We're together. You will not face this alone."

The allies have boosted defense measures in response to Pyongyang's provocations, including by completing the deployment of a US missile defense system in Seongju, South Korea, earlier in the day.

But Mattis refused to be drawn into a question about whether the US would use military action against North Korea.

"You know me. I never speculate on that," he said.

Tensions have run high since North Korea test-fired two intercontinental ballistic missiles in July. On Sunday, it conducted its sixth and most powerful nuclear test, claiming it successfully detonated an H-bomb that can be mounted on a long-range missile.

Experts believe it is only a matter of time before North Korean leader Kim Jong-un acquires the capability to strike the US mainland with a nuclear weapon to deter what he calls US hostility toward the regime. (Yonhap)