The U.S. government has released intelligence alleging that China conducted a secret nuclear test in 2020 at its Lop Nur site, detected as a small 2.75-magnitude seismic event in Kazakhstan.

Christopher Yeaw, assistant secretary for arms control and nonproliferation, described the tremor as “quite consistent” with a nuclear explosion, though independent experts caution that the evidence is limited and inconclusive.

Ben Dando of NORSAR noted the seismic signal could also have been a natural event, given it was recorded at only a single station.

The Comprehensive Nuclear-Test-Ban Treaty Organization confirmed the detection of two small seismic events but said the data alone cannot determine their cause, NPR has reported.

China has strongly denied the allegations, calling the U.S. claims “completely groundless” and accusing Washington of fabricating pretexts for its own nuclear testing.