South Korean media are reporting that work at North Korea's nuclear test site has entered its final stage, heightening speculation that a third atomic test by the communist state is imminent.

The reports, which come from South Korean government source, suggest activity at the nuclear site has increased.

It is believed the assessment is based on satellite images of the area, although any nuclear device will be tested underground.

There are other reports that the tunnel leading to the underground test site has been sealed and that all that is needed is approval to fire from Pyongyang.

North Korea has threatened to detonate a nuclear device in retaliation for new sanctions passed against it by the UN Security Council.