Image copyright KCNA Image caption Kim Jong-un and senior officials confer over North Korea's nuclear weapons programme in late 2017

Chinese scientists have concluded that North Korea's nuclear test site has partially collapsed, potentially rendering it unusable.

The test site at Punggye-ri has been used for six nuclear tests since 2006.

After the last, in September, a series of aftershocks hit the site, which seismologists believe collapsed part of the mountain's interior.

On Saturday, North Korean leader Kim Jong-un announced he was suspending his country's nuclear and missile tests.

The surprise declaration came ahead of historic talks with South Korea and the US.

The latest research from the University of Science and Technology of China (USTC) is due to be published in Geophysical Research Letters, a journal of the American Geophysical Union, in the coming days.

It concluded that eight and a half minutes after September's test, there was "a near-vertical on-site collapse towards the nuclear test centre".

The Punggye-ri site is situated in mountainous terrain in North Korea's north-east, and tests have taken place in a system of tunnels dug below Mount Mantap.

A one-page summary of the research on the USTC website concluded: "The occurrence of the collapse should deem the underground infrastructure beneath mountain Mantap not be used for any future nuclear tests."

But those words do not appear in the final peer-reviewed paper. It instead says that the "collapse in the test site calls for continued close monitoring of any leaks of radioactive materials".

Prof Wen Lianxing, the lead author of the study, told the Wall Street Journal that the conclusion about the test site's viability would not be included in the published paper, but did not say why.

Image copyright USTC Image caption A graphic on the university website concludes the test site is likely unusable - but the peer-reviewed paper does not

The research echoes similar findings by a team from the Jilin Earthquake Agency, published in the same journal last month.

That team concluded that the explosion "created a cavity and a damaged 'chimney' of rocks above it", leading to a collapse.

The earlier paper did not offer an opinion on the viability of the test site in the wake of the collapse.

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A possible tunnel collapse at Mount Mantap has long been suspected, with Chinese scientists expressing concern soon after September's large-scale nuclear test.

The US Geological Survey recorded a second seismic event about eight minutes after the test, which it assessed as a "collapse" of the cavity.

Two aftershocks were detected as late as December, prompting concerns about the stability of the surrounding mountains.