More than 200 people were killed when a tunnel at North Korea's nuclear test site collapsed following its latest test, according to Japanese TV.
About 100 workers were affected by the initial collapse at the Punggye-ri site on 10 September following the country's sixth nuclear test, TV Asahi reported, citing unnamed North Korean sources.
Over 100 more are likely to have died as further collapses occurred during a rescue operation, the broadcaster said.
There has been no independent confirmation of the report.
Experts said the sixth blast, on 3 September, would have destabilised the region.
It was reported two weeks ago that the area surrounding the Punggye-ri nuclear test site, in the country's north west, was suffering from "Tired Mountain Syndrome" as a result of the underground blasts.
The geological condition occurs when underground nuclear blasts cause the surrounding rock to become weak and permeable.
The 3 September test caused the USGS to register a tremor of magnitude 6.3, with a smaller, second earthquake being measured at the site, about eight minutes later.
The USGS said the second quake was indicative of a collapse and it recorded a further seismic event of 3.1 magnitude on the site on 23 September.
The 38 North website, which reports on issues involving North Korea, said the effects of the latest blast could potentially extend as far as 1.4km from the detonation point.
But it said they probably won't stop Mount Mantap being used for further tests.
Other specialists, however, have said the effects on the mountain mean the Punggye-ri site may not be used for much longer to test nuclear weapons.
Meanwhile, Seoul and Beijing have agreed to move beyond a year-long stand-off over the deployment of the US anti-missile THAAD system in South Korea.
The dispute has been devastating South Korean businesses that rely on Chinese consumers and the announcement comes just days before Donald Trump begins a trip to Asia, where the North Korean nuclear crisis will take centre stage.
In Japan, where tensions have been raised as a result of the North Korean nuclear and missile tests, Prime Minister Shinzo Abe has met NATO Secretary-General Jens Stoltenberg.
They discussed cooperation in addressing security challenges, with Mr Stoltenberg telling North Korea afterwards it "must abandon its nuclear ballistic missile programmes and implement a complete denuclearisation of the Korean Peninsula".
