A reported tunnel collapse at North Korea’s key nuclear testing site has sparked fears of a major radiation leak.

At least 100 workers at the Punggye-ri nuclear site are feared to have died in the accident that happened around 10 September following the regime’s sixth atomic test earlier that month, Japanese broadcaster TV Asahi reported.

The broadcaster said there was also a second collapse during the rescue operation which may mean the death toll exceeds 200.

News of the two accidents has not been verified by Pyongyang, which is usually reluctant to acknowledge any failures to the outside world.

North Korea's sixth and largest nuclear test on 3 September triggered landslides in the detonation area according to satellite images taken shortly afterwards, experts said.

News of the accidents came just days as the head of the Korea Meteorological Administration, Nam Jae-Cheol, said future nuclear tests risked the collapse of the mountain test site, which could leak dangerous levels of radiation into the surrounding atmosphere.

He said: “Based on our analysis of satellite imagery, we judge that there is a hollow space, which measures about 60 to 100 metres, at the bottom of Mount Mantap in the Punggye-ri site.”

So far, the nuclear tests are not thought to have caused any radiation damage to foreign countries but scientists warn the hollow space inside the mountain makes it unstable.

Chinese nuclear weapons researcher Wang Naiyan told the South China Morning Post last month: “We call it ‘taking the roof off’. If the mountain collapses and the hole is exposed, it will let out many bad things.

Kim Jong-un inspects weapon North Korea says is powerful hydrogen bomb Show all 6 1 /6 Kim Jong-un inspects weapon North Korea says is powerful hydrogen bomb Kim Jong-un inspects weapon North Korea says is powerful hydrogen bomb Photos released by North Korea show Kim Jong-un talking to subordinates next to a device thought to be the new thermonuclear weapon. There is no way of independently verifying the pictures STR/AFP/Getty Images Kim Jong-un inspects weapon North Korea says is powerful hydrogen bomb North Korea claims it has successfully tested an advanced hydrogen bomb which could be loaded onto an intercontinental ballistic missile AFP/Getty Kim Jong-un inspects weapon North Korea says is powerful hydrogen bomb A diagram on the wall behind Mr Kim shows a bomb mounted inside a cone STR/AFP/Getty Images Kim Jong-un inspects weapon North Korea says is powerful hydrogen bomb North Korean leader Kim Jong-Un (C) attending a photo session with participants of the fourth conference of active secretaries of primary organisations of the youth league of the Korean People's Army (KPA) in Pyongyang STR/AFP/Getty Images Kim Jong-un inspects weapon North Korea says is powerful hydrogen bomb A new stamp issued in commemoration of the successful second test launch of the "Hwasong-14" intercontinental ballistic missile KCNA via Reuters Kim Jong-un inspects weapon North Korea says is powerful hydrogen bomb A new stamp issued in commemoration of the successful second test launch of the "Hwasong-14" intercontinental ballistic missile KCNA via Reuters

“A 100 kiloton bomb is a relatively large bomb. The North Korean government should stop the tests as they pose a huge threat not only to North Korea but to other countries, especially China.”

Pyongyang has stepped up its nuclear and missile tests over the past year as the international community has put further pressure on it.

Donald Trump has repeatedly made incendiary public statements about the conduct of North Korean leader Kim Jong Un.

The US President warned he will invoke “fire and fury” against the country “as you have never seen before” if it did not end its nuclear programme.