Over 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, reports the Independent.

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.

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. 

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.

BDST: 1740 HRS, OCT 31, 2017
AP