As many as 200 North Korean labourers have been killed after a mine shaft being dug at the regime's nuclear test site collapsed, according to Japan's Asahi TV.

Sources in North Korea told the news channel that a tunnel being excavated by around 100 workers at the Punggye-ri test site collapsed earlier this month.

An additional 100 labourers sent to rescue their colleagues were reportedly killed when the tunnel suffered a second collapse.

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.