"It is regrettable that international experts were not invited to witness the site closing," Guterres said in a statement.

Foreign journalists invited at the Pungyye-ri test facility in North Hamgyong province described a series of explosions throughout the day, three of them in entry tunnels to the underground facility, followed by blasts that demolished a nearby barracks and other structures.

Guterres expressed hope that "this confidence-building measure will contribute to ongoing efforts towards sustainable peace and verifiable denuclearization of the Korean Peninsula."

The Pungyye-ri site has been the staging ground for all six of the North's nuclear tests, including its latest and by far most powerful one in September last year, which Pyongyang said was a hydrogen bomb.