More than 200 people were killed when tunnels at North Korea’s Punggye-ri nuclear test site collapsed after the test of a hydrogen bomb last month, Japan’s Asahi TV reported on Tuesday.

The collapse occurred while a tunnel was being constructed at Punggye-ri in the northeastern part of the country this month, the channel cited North Korean sources as saying.

About 100 workers were killed in the initial accident and there was a collapse during a rescue operation, resulting in a death toll of more than 200, the report said.

The test site was affected by North Korea’s sixth nuclear blast on September 3 involving a 100-kiloton hydrogen bomb, which was about seven times more powerful than the atomic bomb dropped by the US on Hiroshima in 1945.

“It is believed that the ground was loose due to the sixth nuclear test conducted on September 3,” Asahi TV reported.

The Korea Meteorological Agency said the test with the hydrogen bomb had created a crater of 60 metres to 100 metres under the mountain where there is a nuclear test site.

The agency had also said there is a possibility that radioactive substances may leak out in the future if more nuclear tests are conducted at the site.

Experts had said a series of tremors and landslides near the Punggye-ri nuclear test site meant the sixth and largest blast has destabilised the region, and the site might not be used for much longer to test nuclear weapons.

The series of quakes prompted experts to suspect the test of the hydrogen bomb may have damaged the mountainous location where all of North Korea’s six nuclear tests were conducted.

South Korea’s National Intelligence Service has said that North Korea was possibly preparing two new tunnels for tests after the test of the hydrogen bomb. The tunnel used for the first nuclear test had been shut down, while a second tunnel had been used for the next five tests.

This second tunnel could have collapsed after the test of the hydrogen bomb, experts said.