About 100 people are said to have been trapped inside the unfinished tunnel at the Punggye-ri nuclear test site.
It is not known why exactly the disaster occurred, or the exact numbers of fatalities.
Experts have warned that the site may have become unstable after a series of weapons tests.
South Korea said that another nuclear blast could trigger a collapse of the mountainous site and a leak of radioactive materials.
Earlier this month monitoring agency 38 North said that the launch site was suffering from ‘Tired Mountain Syndrome’ – a geological condition that occurs when underground nuclear blasts cause the surrounding rock to become weak and permeable.
Five of the country’s six nuclear tests have been carried out at this location.
Earlier this week North Korea ramped up its rhetoric by stating that the promise of a nuclear bomb test should be taken “literally”.
The country’s foreign minister, Ri Yong Ho, said last month that Pyongyang was considering conducting “the most powerful detonation” of a hydrogen bomb over the Pacific ocean.
That threat, made after Donald Trump vowed to “totally destroy” North Korea if they continued their nuclear programme, has been given more weight by Ri Yong Pil, a senior diplomat in North Korea’s Foreign Ministry.
He told CNN: “The foreign minister is very well aware of the intentions of our supreme leader, so I think you should take his words literally.”
