"The Nuclear Weapons Institute of the DPRK held a ceremony for completely dismantling the northern nuclear test ground on May 24... to ensure transparency of the discontinuance of nuclear test," the institute said in an English language statement carried on the state-run KCNA news agency.

DPRK is the abbreviated version of the country's official name.

"Dismantling the nuclear test ground was done in such a way as to make all the tunnels of the test ground collapse by explosion and completely close the tunnel entrances, and at the same time, explode some guard facilities and observation posts on the site," the statement added.

The institute said two of the tunnels in the mountain were "ready for use for carrying out very powerful underground nuclear tests at any time" before they were destroyed.

No leakage of radiation had been detected at the site during the demolition, the statement added.