A tunnel near the North Korean nuclear test site collapsed recently trapping 200 people who are feared dead besides sparking concerns of radiation leakage.
About 100 people were trapped inside when the unfinished tunnel at the North's Punggye-ri nuclear test site collapsed, and an additional 100 people could have been killed while trying to rescue those trapped as a second collapse occurred, Japan's TV Asahi reported.
The broadcaster said the collapse took place around 10 September.
Monitors have previously picked up seismic shocks at North Korea's main nuclear testing site that are consistent with a major collapse.
Experts have said a series of tremors and landslides near the nuclear test base probably mean the country's sixth and largest blast on 3 September has destabilised the region, and the Punggye-ri nuclear site may not be used for much longer to test nuclear weapons.
Chinese scientists have warned that if the whole mountain collapsed, radiation could escape and drift across the region.