North Korea’s main nuclear test site has partially collapsed, making it unsafe for further use, a group of Chinese geologists has found, according to AP. The findings raise questions over Pyongyang’s motivation behind the announcement that it will cease its nuclear programme for now.

North Korean leader Kim Jong-un had announced on Saturday that he was suspending all missile tests and will shut down a nuclear test site ahead of talks with South Korea and the United States. US President Donald Trump had praised the move and called it “big progress”.

The research by the scientists at the University of Science and Technology of China showed that the mountain above the Punggye-ri test site has collapsed and needs to be monitored for leaking radiation. The test site is less than 100 km from North Korea’s border with China.

The researchers’ findings are based on data collected after the nuclear test on September 3, 2017. The test was the most powerful of North Korea’s six nuclear device tests, and is said to have triggered four earthquakes in the following weeks.

North Korea’s nuclear tests at the Punggye-ri test site brought the region to the brink and soured ties with the United States and Japan last year. The missile fired by North Korea in the September test had flown over Japan before landing in the Pacific Ocean.