Collapse at N. Korea nuclear test site 'leaves 200 dead'London, Oct 31 (AZINS) As many as 200 North Korean labourers were killed after a mine shaft being dug at a nuclear test site collapsed, the media reported.

The Telegraph quoted sources in North Korea saying that a tunnel being excavated by around 100 workers at the Punggye-ri test site crashed in early October.

An additional 100 labourers sent to rescue their colleagues were reportedly killed when the tunnel suffered a second collapse, according to Japan's Asahi TV.

An exact date for the disaster has not been provided, but it comes shortly after North Korea conducted its sixth - and most powerful - underground nuclear test at the site.

North Korea claims the September 3 test beneath Mount Mantap was of a hydrogen bomb, with monitors suggesting the detonation was equivalent to an earthquake with a magnitude of 6.1 on the Richter scale.

Some analysts put the yield of the weapon as high as 280 kilotons, while seismologists picked up signs of underground collapses in the hours and days after the blast.

Satellite images of the Punngye-ri site taken immediately after the test revealed significant damage to surface features, including landslips.

On October 17, a study published by the US-Korea Institute at Johns Hopkins University and published on the 38 North web site suggested the sixth underground test at the site had caused "substantial damage to the existing tunnel network under Mount Mantap".

It added that there was a possibility that the site was suffering "Tired mountain syndrome", although there were no indications that it was being abandoned for future nuclear tests.

Nam Jae-chol, head of South Korea's Meteorological Administration, warned on Monday that further tests at Punggye-ri could cause the mountain to collapse and release radioactivity into the environment.

Chinese scientists have issued similar warnings, suggesting that nuclear fallout could spread across "an entire hemisphere" if the mountain did collapse.