Foreign journalists at the Punggye-ri nuclear testing site said they witnessed a huge explosion.
The North offered to scrap the site earlier this year as part of a diplomatic rapprochement with South Korea and the US.
But scientists believe it partially collapsed after the last test in September 2017, rendering it unusable.
International inspectors were not allowed to attend the dismantling of the Punggye-ri site in the mountainous north-east of the country.
Sky News’ Tom Cheshire was among a group of foreign journalists invited to witness the event. He said: “We hiked up into the mountains and watched the detonation from about 500m away.
“They counted it down – three, two, one. There was a huge explosion, you could feel it. Dust came at you, the heat came at you. It was extremely loud.”