A study by seismologists from the University of Science and Technology of China suggested rock had collapsed under Mount Mantap in the country's northeast after a nuclear test in September that triggered a 6.3-magnitude earthquake.

An English-language abstract by the study authors in another section of the university's website concluded: "The occurrence of the collapse should deem the underground infrastructure beneath mountain Mantap not be used for any future nuclear tests."

But Jeffrey Lewis of the Middlebury Institute of Strategic Studies tweeted Thursday: "The mountain did not collapse and there is no evidence that it is unusable."

"I checked an image from @planetlabs dated April 25. The mountain is still there," quipped Lewis.

Ahead of a hotly awaited inter-Korean summit on Friday, the North's leader Kim Jong Un declared he would shut down the Punggye-ri testing site, which includes Mount Mantap.

Sceptics have said it was an empty concession by Kim as the site is already suffering from "tired mountain syndrome" and may be obsolete.

But Lewis noted that while the explosion did cause a cavity to collapse, "this does not mean the tunnel complex below the mountain is ruined".

All six of the North's tests have been carried out at Punggye-ri, and he stressed that North Korea had bored tunnels into other mountains at the site that would not be compromised by the explosion under Mantap.

"At most, North Korea might shift big tests to neighbouring mountains," said Lewis, concluding: "Kim has agreed to stop nuclear testing because of the summit(s), not because his nuclear test mountain collapsed."

Earlier, the respected 38 North website, which regularly examines satellite photography of Punggye-ri, also noted that tunnelling had been observed up to March.

It was continuing at a reduced level earlier this month, suggesting that "either the tunnel was complete and ready for future renewed testing or that the slowdown simply mirrored the ongoing political changes underway."

"There is no basis to conclude that the Punggye-ri nuclear test site is no longer viable for future nuclear testing," the commentary added.

"It is not a case of passing off damaged goods."