A senior U.S. State Department official on Tuesday disclosed additional evidence supporting U.S. allegations that China conducted a covert underground nuclear test in June 2020, amid mounting concerns over the erosion of global arms control frameworks.
Speaking at an event hosted by the Hudson Institute, Assistant Secretary of State Christopher Yeaw cited newly reviewed seismic data recorded by a remote monitoring station in Kazakhstan. The station detected a magnitude 2.75 event approximately 450 miles from China’s Lop Nur nuclear test site on June 22, 2020.
“I’ve looked at additional data since then. There is very little possibility I would say that it is anything but an explosion, a singular explosion,” Yeaw said, noting that the seismic signature was inconsistent with mining activity or natural earthquakes. “It is … what you would expect with a nuclear explosive test.”
Yeaw further asserted that the event bore hallmarks of “decoupling,” a technique in which a nuclear device is detonated within a large underground cavity to dampen seismic signals and hinder detection.
Earlier this month, Under Secretary of State for Arms Control Thomas DiNanno publicly accused China of conducting concealed nuclear explosive tests and employing measures to obscure seismic evidence.
“Today, I can reveal that the U.S. Government is aware that China has conducted nuclear explosive tests, including preparing for tests with designated yields in the hundreds of tons,” DiNanno said.
The allegations come as international monitoring authorities urge caution. The Comprehensive Nuclear-Test-Ban Treaty Organization (CTBTO), which oversees a global network of seismic stations, confirmed that its Kazakhstan facility recorded “two very small seismic events” 12 seconds apart on June 22, 2020.
Executive Secretary Robert Floyd stated that the events were well below the network’s detection threshold of 500 metric tons of TNT equivalent, and that available data alone do not permit definitive conclusions regarding their origin.
China, a signatory but not a ratifier of the Comprehensive Nuclear-Test-Ban Treaty, has denied the U.S. allegations. Beijing’s last acknowledged underground nuclear test took place in 1996.
The United States also signed but did not ratify the treaty, and has not conducted an underground nuclear test since 1992, instead relying on advanced simulations and supercomputing to maintain its arsenal.
The dispute unfolds against the backdrop of intensifying strategic competition and the expiration of key arms control agreements.
President Donald Trump recently urged China to join trilateral arms control negotiations with Russia following the lapse of the New Strategic Arms Reduction Treaty (New START) on Feb. 5. Beijing declined, citing the comparatively smaller size of its nuclear arsenal.
According to U.S. Department of Defense estimates, China possesses more than 600 operational nuclear warheads, a figure projected to surpass 1,000 by 2030. The Federation of American Scientists estimates Russia maintains approximately 5,459 warheads, while the United States holds roughly 5,177.
With New START’s expiration removing formal caps on deployed strategic warheads and delivery systems, Washington and Moscow have indicated they will continue to observe its limits on an informal basis, even as broader arms control mechanisms face increasing strain.
