"It doesn't look like a nuclear explosion, but we are not a hundred percent sure," Mohamed ElBaradei, director general of the International Atomic Energy Agency, told CNN television.

"I think it is unlikely, but we are not there, and we cannot really validate this conclusion for sure," he added.

North Korean Foreign Minister Paek Nam-Sun told British officials his country's engineers blew up a mountain near the border with China to prepare for a hydro-electric project.

He said the explosion was intentional and non-nuclear.