As many as 200 workers could have been killed after a tunnel under construction at North Korea’s nuclear test site collapsed, Japanese news reports said on Tuesday.

The Yohap News Agency said about 100 people were trapped inside the unfinished tunnel at the Punggye-ri nuclear test site when the incident happened. 

Japan’s TV Asahi’s said an additional 100 people could have been killed while trying to rescue those trapped south of the Mantapsan mountain.

Reports have not detailed exactly when the incident happened – although one official has said the collapse happened on October 10 – or given a confirmed death toll. 

The Yohap report suggested experts had warned the test site must have become “fatigued and unstable” following six nuclear tests including the most recent one in September, which featured a 100-kiloton hydrogen bomb.

The series of small-scale earthquakes followed the test and it is has been claimed the tunnel may have been constructed to move the test site to another part of the mountain. 

“If North Korea were to attempt to continue testing under this mountain (such as in the area more to the eastern side), then we would expect to see new tunneling in the future near the North Portal, still under Mt. Mantap,” researchers Frank Pabian and Jack Liu wrote in a report published earlier this month on the North Korea monitoring website 38 North

On Monday, the chief of South Korea’s weather agency Korea Meteorological Administration, Nam Jae-cheol, said during a parliamentary meeting that another nuclear blast could trigger a collapse of the North’s mountainous test site and a leak of radioactive materials, the agency said.