A tunnel at North Korea's main nuclear test site collapsed earlier this month, leaving hundreds of workers dead, according to a Japanese television news report Tuesday, prompting fears about the safety of the facility.

Citing the report on the Asahi TV channel, South Korea's Yonhap news agency said the incident occurred at the Punggye-ri facility in northeastern North Korea when an unfinished tunnel collapsed, fatally trapping 100 people inside. A further 100 people reportedly died after they rushed to the site and a second collapse occurred.

The South Korean news agency said the incident occurred on October 10.

Experts have been warning North Korea that its mountainside nuclear test site must have become fatigued and unstable after six nuclear tests – including one H-bomb test on September 3 that was its most powerful yet – and that a collapse was possible at any time.

The South China Post Morning Post said Chinese geologists had warned North Korean scientists last month that the nuclear tests could lead to the collapse of the mountain at the site.

Open gallery view South Koreans watching a TV news program reporting on an earthquake in northeastern North Korea, September 23, 2017. Credit: Ahn Young-joon/AP

British newspaper the Daily Express said the Institute of Geology and Geophysics, part of the Chinese Academy of Sciences, issued a further statement this week, warning: “China cannot sit and wait until the site implodes. Our instruments can detect nuclear fallout when it arrives, but it will be too late by then.

“There will be public panic and anger at the government for not taking action," it stated, adding, "The fallout can spread to an entire hemisphere."

South Korean scientists are also concerned that a collapse at the mountainsite could trigger a radiation leak.

South Korea’s chief meteorologist, Nam Jae-Cheol, told a meeting of politicans on Monday that any future nuclear test at the site could cause a collapse, the Associated Press reported.

It added that a Korea Meteorological Administration researcher, Lee Won-Jin, presented an analysis of satellite images indicating that landslides occurred near the facility following the September 3 test.

The Daily Express quoted a seismologist at Columbia University's Lamont-Doherty Earth Observatory, Paul Richards, as saying: “What we are seeing from North Korea looks like some kind of stress in the ground. In that part of the world, there were stresses in the ground, but the explosions have shaken them up," he noted.

North Korea's nuclear tests have grown in ambition in recent months, much to the concern of Western states. U.S. President Donald Trump ratcheted up the tension after declaring at the UN General Assembly in September, "The United States has great strength and patience, but if it is forced to defend itself and its allies, we will have no choice but to totally destroy North Korea," Trump said. "Rocket Man is on a suicide mission for himself and for his regime," he added, referring to regime leader Kim Jong Un.