As many as 200 construction workers are feared dead after the collapse of a tunnel at a nuclear test site in North Korea, according to a Japanese media report.

The tunnel was being built at the Punggye-ri test site when it collapsed, according to a report on Japan's TV Asahi citing an unnamed source in North Korea.

It said that about 100 people were initially trapped in the tunnel and another 100 may have been killed by a second collapse as they tried to rescue the first group.

The broadcaster said the collapse took place around 10 September.

South Korea's Yonhap news agency said the report did not provide further details, including when the incident occurred.

Monitors have previously picked up seismic shocks at North Korea's main nuclear testing site that are consistent with a major collapse.

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

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The mountain visibly shifted during the last nuclear test, which was recorded as a 6.3 magnitude earthquake. Since then, the area, which is not known for natural seismic activity, has had three more quakes.

Chinese scientists have warned that if the whole mountain collapsed, radiation could escape and drift across the region.

It comes amid news North Korea has conducted mass evacuation drills as it prepares for the possibility of war.

South Korean media said drills had been conducted in "secondary and tertiary cities and towns" over the last week, mostly on the pariah state's east coast, which borders the Sea of Japan.

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

Blackout drills, where towns turn off all light sources at night to avoid illuminating targets for the enemy, were also conducted.

The threat North Korea launching a nuclear missile attack is accelerating, US Defence Secretary James Mattis has said.

General Mattis accused the North of illegal and unnecessary missile and nuclear programmes and pledged to repel any strike.

"North Korea has accelerated the threat that it poses to its neighbors and the world through its illegal and unnecessary missile and nuclear weapons programs," he said, adding that US-South Korean military and diplomatic collaboration had thus taken on "a new urgency."