Pyongyang announced its plan to "completely" dismantle the Punggye-ri facility in the country's northeast, inviting some foreign journalists to witness the destruction.

"There was a huge explosion, you could feel it. Dust came at you, the heat came at you. It was extremely loud," Tom Cheshire, a journalist for Sky News who was among those invited to attend the ceremony, wrote on the British broadcaster's website.

Yonhap news agency, citing South Korean pool reporters at the scene, said multiple explosions were heard throughout the day, beginning at 11am (0300 GMT) until 4.17pm. 

Punggye-ri 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 an H-bomb.

Experts are divided over whether the demolition will render the site useless. Sceptics say the facility has already outlived its usefulness with six successful nuclear tests in the bag and can be quickly rebuilt if needed.

North Korea also did not invite any independent observers from overseas.

But others say the fact that North Korea agreed to destroy the site without preconditions or asking for something in return from Washington suggests the regime is serious about change.