More than 200 people are believed to have died when a tunnel at North Korea’s main nuclear test site collapsed in September after the country’s sixth and most powerful atomic test — an accident that could have far-reaching implications for the North’s nuclear weapons program.

The tunnel at the Punggye-ri test site was undergoing construction around Sept. 10, a week after the Sept. 3 nuclear explosion, when an initial collapse killed about 100 workers, Japan’s TV Asahi reported Tuesday, citing unnamed sources in the isolated country. After rescuers were sent in, a second collapse left at least 100 more feared dead.

The Japan Times could not confirm the veracity of the report. North Korea rarely acknowledges major accidents, and any accident involving the country’s ever-improving nuclear program would likely be off-limits for state-run media.

The North’s sixth test — which Tokyo estimated had an explosive yield of 160 kilotons, more than 10 times the size of the Hiroshima bomb — and several aftershocks at the site are believed to have triggered the collapse.

Frank V. Pabian, an analyst with the North Korea-watching website 38 North, said Wednesday that such a scenario is plausible.

“The aftershocks could well have caused tunnel collapses,” Pabian wrote on Twitter, adding that the North “most certainly could have had personnel in the tunnels working during any one of the next three aftershocks.”

On Monday, South Korea’s weather agency, the Korea Meteorological Administration, told a parliamentary hearing that another nuclear blast could trigger a collapse at the mountainous test site, leading to a dangerous leak of radioactive materials.

Experts have warned that the nuclear-armed country’s six atomic tests at the site have destabilized the area and that it may not be usable for future tests.

But others say its abandonment for nuclear testing should not be expected.

“Even in the face of what has been dubbed ‘Tired Mountain Syndrome,’ abandonment of the site for nuclear testing should not be expected,” 38 North said in an Oct. 17 analysis, referring to a condition in which underground nuclear testing fractures and weakens rock, increasing permeability and the risk of radioactive contamination into the environment.

“Historical precedent, combined with the presence of two other, as yet unused tunnel complexes within the test site, leads us to conclude that there is no valid reason to assume that the Punggye-ri test site is unable to contain additional underground nuclear tests,” the report went on.

Pabian said Wednesday that the reported collapses could change that calculus.

“If true, that will have a significant impact on future tunneling under Mt. Mantap and could well lead to an abandonment of the North Portal at least and perhaps delay further activity there for the foreseeable future,” Pabian said in a tweet. Mantap, where the tunnels were dug, is located inside the Punggye-ri nuclear complex.

“Only time will tell, but we will be watching closely.”

North Korean leader Kim Jong Un has called his country’s nuclear weapons program a “treasured sword” meant to protect it from aggression.

Despite calls for the complete denuclearization of the Korean Peninsula, Kim has ramped up the testing of missiles and nuclear weapons — including September’s claimed hydrogen bomb blast — and has vowed never to give up his arsenal.

U.S. President Donald Trump and Prime Minister Shinzo Abe have ratcheted up a program of “maximum pressure” meant to bring the recalcitrant leader to the negotiating table on their terms.

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