At least 200 people have been killed at a nuclear test site in North Korea after a tunnel collapsed, according to an unverified Japanese media report.

The collapse is said to have taken place during the construction of a new underground facility at the Punggye-ri site in northeastern North Korea, the report says.

But there has been no official confirmation of the claims, apparently made by an unnamed North Korean 'source'.

According to Japan's TV Asahi, up to 100 people had been trapped in the tunnels and a further collapse happened during attempts to rescue them, raising the death toll to at least 200.

The incident is believed to have happened on October 10, but another news agency, South Korea's Yonhap, said it was still unclear when the disaster took place.

The collapse is said to have taken place during the construction of a new underground facility at the Punggye-ri site (pictured) in northeastern North Korea, the report says

At least 200 people have been killed at a nuclear test site in North Korea after a tunnel collapsed, Japanese media has claimed. Dictator Kim Jong-un is pictured

The last five of Pyongyang's six nuclear tests have all been carried out at the Punggye-ri nuclear test site (pictured) under Mount Mantap, in the north-west of the country

The network claimed the incident was the result of the weakening of the ground surrounding the site in the wake of North Korea's sixth nuclear test which was carried out at the same site.

It comes a day after Seoul warned that one more North Korean nuclear detonation could destroy its mountain test site and trigger a radiation leak.

South Korea says any future nuclear test by Kim Jong-un risks collapsing the location set aside for launching missiles.

Overhead pictures taken in April show some of the facilities at North Korea's Punggye-ri nuclear test

The launch of a Hwasong-12 missile on September 16, 2017. A launch like this at one North Korean test site could destroy it and trigger a radiation leak

Seoul detected several earthquakes near the hermit nation's nuclear test site in the country's northeast after its sixth and most powerful bomb explosion in September.

Experts say the quakes suggest the area is now too unstable to conduct more tests there.

South Korea's weather agency chief Nam Jae-Cheol made the comments Monday during a parliament committee meeting.

He was responding to a lawmaker's question about whether another North Korean test could lead to such an accident.

Earlier this month, US experts issued a similar warning, stating a second nuclear test site used by North Korea in the country's north west could cave in but that it won't be abandoned.

Five of Pyongyang's recent tests have been carried out under Mount Mantap at the Punggye-ri military base, which is located in the north west of North Korea.

But now the base is said to be suffering from 'Tired Mountain Syndrome' after three small earthquakes occurred nearby after the blasts.