A tunnel at North Korea's nuclear test site collapsed after Pyongyang's sixth atomic test in September, possibly killing more than 200 people, Japanese broadcaster TV Asahi said on Tuesday, citing unnamed sources familiar with the situation.

Reuters has not been able to verify the report.

About 100 workers at the Punggye-ri nuclear site were affected by the initial collapse, which took place around Sept. 10, the broadcaster said.

A second collapse during a rescue operation meant it was possible the death toll could have exceeded 200, it added.

Experts have said a series of tremors and landslides near the nuclear test base probably mean the country's sixth and largest blast on Sept. 3 has destabilised the region, and the Punggye-ri nuclear site may not be used for much longer to test nuclear weapons.