North Korea nuclear test site part-collapsed: Chinese experts

North Korea's underground nuclear test site has partially collapsed following a massive bomb blast last year, making it unusable, Chinese seismologists have concluded.

The North's leader Kim Jong Un declared last week that his regime would halt nuclear and long-range missile tests and shut down its nuclear site at Punggye-ri under Mount Mantap in the country's northeast.

The offer came days before his summit this Friday with the South's President Moon Jae-in, which is scheduled to be followed by a summit with US President Donald Trump.

North Korea conducted five of its six nuclear tests at the site, with the biggest blast last September 3 triggering a 6.3-magnitude earthquake that was felt across the northern border with China.

The North claimed it tested a hydrogen bomb.

Landslides and earthquakes following the explosion led to speculation that the site was suffering from "tired mountain syndrome".

Two studies involving Chinese experts have found that a 4.1-magnitude aftershock that took place 8 1/2 minutes after the first quake caused the collapse of rock inside the mountain.

"It is necessary to continue monitoring possible leaks of radioactive materials caused by the collapse incident," said the University of Science and Technology of China in a summary of one study posted on its website.

The university said the study would be published in Geophysicial Research Letters, a journal of the American Geophysical Union.

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."