The United States does not want war with North Korea, the US military chief said on the demilitarised zone metres away from the communist state, as he warned Pyongyang to stop threatening "catastrophe" with its nuclear weapons.

James Mattis made the brief comments standing next to South Korean Defence Minister Song Young-moo on Friday, describing Kim Jong-un's leadership as an "oppressive regime" that mistreats its people while its neighbour to the south offers a vibrant democracy.

"Our goal is not war but rather the complete, verifiable, and irreversible denuclearisation of the Korean Peninsula," Mattis was quoted as saying by South Korea's Yonhap news agency at Panmunjom village.

The United States has about 30,000 American troops stationed in South Korea, a remnant of the 1950-1953 war on the peninsula that has never officially ended, with only an armistice agreement signed.

Song said North Korea can never use its nuclear weapons. "If it does, it will face retaliation by the strong combined force of South Korea and the US," he warned.

North Korea must to return to dialogue between the two Koreas, Song added.

The threat of nuclear war has escalated in recent months as President Donald Trump has intensified his rhetoric against Kim's regime, including saying he would "totally destroy" North Korea if it threatened the US or its allies.

Pyongyang responded by threatening to detonate a nuclear weapon in the atmosphere after its sixth and most powerful underground nuclear test last month.