The United States has disclosed new details suggesting China may have conducted an underground nuclear test in June 2020.

The claim, based on seismic readings, comes amid heightened tensions over global arms control and nuclear monitoring.

Assistant Secretary of State Christopher Yeaw told the Hudson Institute that seismic data from a remote station in Kazakhstan indicated an explosion of magnitude 2.75 near the Lop Nor test site in western China on June 22, 2020.

Yeaw, a nuclear engineering expert and former intelligence analyst, said the data were inconsistent with earthquakes or mining activity. “I’ve looked at additional data since then. There is very little possibility I would say that it is anything but an explosion,” he said.

He also suggested China may have used a decoupling method to mask the blast’s size, a technique where a device is detonated in a large underground chamber to reduce detectable shockwaves.

International monitoring

The Comprehensive Test Ban Treaty Organization (CTBTO), responsible for global nuclear monitoring, said the available data were insufficient to conclusively determine whether a nuclear test occurred. Robert Floyd, the CTBTO executive secretary, explained that two small seismic events recorded at the time were far below the threshold needed to confirm a nuclear detonation.

China has firmly denied any nuclear testing in 2020. Spokesperson Liu Pengyu called the U.S. claims “entirely unfounded” and accused Washington of political manipulation to justify its own nuclear ambitions.

“China urges the U.S. to reaffirm the five nuclear-weapon states' commitment on refraining from nuclear tests and uphold the global consensus against nuclear tests,” Liu said.

Strategic context

The revelations come as U.S. President Donald Trump pushes China to join Washington and Moscow in negotiating a replacement for the New START treaty, which expired on February 5. The lapse has raised concerns over a potential acceleration in the global nuclear arms race.

China currently has more than 600 operational warheads and is expanding its strategic nuclear arsenal, with projections suggesting more than 1,000 warheads by 2030, according to the Pentagon.

Both the U.S. and China have signed, but not ratified, the Comprehensive Nuclear-Test-Ban Treaty, which legally obliges both nations to refrain from testing. The U.S. last conducted an underground nuclear test in 1992 and now relies on simulations to maintain its arsenal.