Damage to North Korea's nuclear testing facility has caused it to collapse, according to Chinese scientists, casting new light on Pyongyang's offer to halt its nuclear testing programme and close the site.
However, the conclusions of the Chinese have been disputed by US experts who point to evidence that the Punggye-ri site in the north-west of North Korea is still operational.
The paring back of Pyongyang's nuclear programme is a key bargaining chip for North Korean supreme leader Kim Jong Un when he meets South Korean president Moon Jae-in at a historic summit on Friday. The topic will also be the focus of talks between Mr Kim and US president Donald Trump at a summit in May or June.
Two groups of Chinese experts said the testing site at Punggye-ri, under a mountain near China's border, caved in after a test on September 3 and was leaking radiation.
The test detonated a 100-kilotonne bomb that excavated a cavern 200 metres in diameter and opened a hole in the surface of Mount Mantap, the peak under which the facility is located, they said.
"In view of the collapse of the North Korea nuclear test site . . . it was necessary to continue to monitor the leakage of radioactive materials that may have been caused by the collapse event," said a statement by geophysicists led by Wen Lianxing at the University of Science and Technology of China in Hefei.
Mr Wen's team findings are expected to be published next month in the journal Geophysical Research Letters, Hong Kong's South China Morning Post reported.
Another Chinese study, published last month in Geophysical Research Letters by a team led by Liu Junqing of the Jilin Earthquake Agency said: "The explosion created a cavity and a damaged 'chimney' of rocks above it."
However, the US-Korea Institute at Johns Hopkins School of Advanced International Studies said on Monday that it was wrong to conclude the test site had been destroyed and was inoperative.
"There remain two portal areas located in more pristine competent rock that can be used for future tests if Pyongyang were to give the order," it said.
Mr Kim said last Saturday that North Korea's nuclear testing programme was "complete" and the facility would be closed.
"Under the proven condition of complete nuclear weapons, we no longer need any nuclear tests, mid-range and intercontinental ballistic rocket tests, and that the nuclear test site in northern area has also completed its mission," Mr Kim was quoted as saying by the state-run Korean Central News Agency.
The US and South Korea welcomed Mr Kim's offer of halting nuclear testing, which they said was a sign of progress and a promising start to upcoming talks.
Damage to Punggye-ri is likely to have dealt a huge blow to the country's nuclear weapons programme, experts said.
However, they also pointed out instances when North Korea had faked damage to its nuclear weapons infrastructure in an effort to win wriggle room from the international community on sanctions. In 2008, North Korea blew up a cooling tower in its Yongbyon reactor complex and invited international media to view the event.
"It was a largely symbolical gesture: a cooling tower is neither expensive nor technologically difficult to build," said Andrei Lankov, a specialist on North Korea at Seoul's Kookmin University. However, he added that he could not say with any certainty what had happened at Punggye-ri.
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