North Korea has concealed the entrance to an underground nuclear site to prevent satellite monitoring, according to South Korean intelligence sources.

Imagery of the Punggye-ri nuclear test facility in North Hamgyong Province seen by the South Korean officials suggests that covers have been placed over a tunnel entrance.

There is widespread international concern at North Korea's threat to attempt a nuclear detonation - its third since 2006.

Both South Korea and the US are reported to have repositioned intelligence monitoring assets to gain a clear a picture as possible of Pyongyang's intentions.