North Korean leader Kim Jong Un
North Korean leader Kim Jong Un guides the launch of a Hwasong-12 missile in this undated photo released by North Korea's Korean Central News Agency (KCNA) on September 16, 2017. KCNA via Reuters

Up to 200 people were reportedly killed after a tunnel at an underground North Korean nuclear site supposedly collapsed. One hundred people were said to have been trapped by the initial tunnel collapse, and another 100 lost in a second collapse while a rescue operation took place.

Japan’s TV Asahi reports that the collapse took place at the Punggye-ri nuclear test site northeast of the country. It was said to have taken place about a week after the test of a hydrogen bomb, the most powerful nuclear explosion ever achieved by the country. The news prompted fears of a massive radioactive leak.

A North Korean reportedly official said the collapse occurred during the construction of an underground tunnel. Reports also claimed that 100 people were trapped and another 100 people were missing.

The Japanese report is yet to be verified. But North Korean analyst and global security expert Ankit Panda expressed doubts. On Twitter, he wrote that he was “highly skeptical of this report that's circulating today.”

Some experts say the ground may have been loosened after the latest nuclear test in early September. South Korea’s Meteorological Agency said there “seems to be a cavity of 60 meters to 100 meters [200 to 330 feet] under the mountain where there is a nuclear test site.”

There were warnings that radioactive substances might leak out if more tests were performed within Mount Mantap. Chinese scientists chimed in, saying further nuclear tests could cause the mountain to collapse.

The Washington Post previously reported that North Korea’s underground nuclear tests were so huge they have “altered the geological structure of the land” and that it’s suffering from “tired mountain syndrome.” Paul Richards, a seismologist at Columbia University’s Lamont-Doherty Earth Observatory, told the news organisation that what they were seeing from North Korea looks like a kind of stress in the ground.

Kim Jong Un’s regime said after the most recent nuclear test on September 3 that North Korea had successfully set off a hydrogen bomb. The country's analysis website 38 North’s Frank Pabian said it was possible the sixth nuclear test could have caused tunnels to collapse.

The report was not confirmed by South Korea’s unification ministry spokeswoman Lee Eugene. She said they were aware about the report but they do not know anything about it.