At least 200 North Korean workers were reportedly killed when a tunnel at the country's nuclear test site collapsed, a Japanese media outlet has said. The tunnel was thought to be under construction when it crumbled on the workers.

The report by Japan's TV Asahi has not been independently confirmed, but there have been warnings in the past about the danger posed by North Korea's Punggye-ri nuclear test site.

Following Pyongyang's sixth and most powerful nuclear test in September 2017, experts had voiced serious concerns that the mountain would cave in if more detonations take place at the facility.

TV Asahi reported on Tuesday, 31 October, that one of the tunnels collapsed, killing 100 North Korean workers initially.

When others were sent in to undertake rescue work, another section of the unfinished tunnel also came down, killing 100 more people. The report, cited by South Korea's Yonhap news agency, does not mention when the alleged collapse took place.

It is nearly impossible to verify such reports from North Korea as the hermit kingdom does not permit outside media outlets to operate inside the country.

Speculation about a possible tunnel collapse emerged shortly after the latest nuclear test, carried out on 3 September.

There was also a report in South Korea which said residents near the nuclear test site were banned from travelling to other parts of the country due to the fear of radiation leak caused by the tunnel collapse.

An earthquake of magnitude 3.4 was detected close to the Punggye-ri test site on 23 September, setting off speculation about how stable the site was.

News about the reported collapse comes just a day after Korea Meteorological Administration's chief warned that a new nuclear test would bring down the facility – where Pyongyang has conducted six detonations since 2006.