The disaster has prompted fears of a massive radioactive leak which could spark a Chernobyl- or Fukushima-style disaster.
A North Korean official said the collapse happened during the construction of an underground tunnel, South Korea’s Yonhap news agency reports.
Some 100 people are said to have been trapped by the initial tunnel collapse, with a further 100 lost in a second collapse during a rescue operation, Asahi reported Tuesday.
Lee Eugene, a spokeswoman at South Korea's unification ministry, said: "We are aware of the report but do not know anything about it."
The accident is believed to have been caused by Kim Joing-un’s sixth nuclear test on March 3 which weakened the mountain, according to the report.
It was reported earlier this year that the mountain under which the base is believed to be hidden was at risk of collapsing and leaking radiation into the region.
Experts said if the peak crumbles, clouds of radioactive dust and gas would blanket the region, the South China Morning Post reported.
The Punggye-ri test site is carved deep into the side of Mount Mantap.
Geophysicist Wen Lianxing and his team at the University of Science and Technology of China in Hefei, Anhui province, said they were “confident” underground detonations were occurring underneath the mountain.
They posted an analysis of data collected from more than 100 seismic monitoring sites across China.
This has narrowed down the location of Pyongyang’s nuclear tests with a margin of error of just 100m. They’ve all been under the same mountain.
Seismic data showed the underground test triggered an earthquake of magnitude 6.3, around ten times more powerful than the fifth test a year ago. READ MORE
