US Accuses China of Secret Nuclear Test as Key Arms Treaty Lapses

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Washington: The United States has accused China of conducting a secret nuclear weapons test in 2020, a claim Beijing strongly denies, as Washington signals a shift toward expanding its nuclear posture following the expiration of its last major arms control treaty with Russia.

The allegation was made on Friday by US Undersecretary of State for Arms Control and International Security Thomas DiNanno during remarks at a global Disarmament Conference in Vienna. DiNanno said the US government believes China carried out a low-yield nuclear explosive test on June 22, 2020, producing a yield “in the hundreds of tons,” without providing further technical details.

The claim comes one day after the New START Treaty, which is the last remaining nuclear arms control agreement between the United States and Russia, formally expired, leaving the world’s two largest nuclear powers without limits on deployed strategic nuclear weapons for the first time in decades.

DiNanno alleged that China attempted to conceal the test by using “decoupling,” a technique that reduces seismic signals by detonating a nuclear device inside a large underground cavity. He said such actions violated China’s test ban commitments.

However, the Comprehensive Nuclear-Test-Ban Treaty Organisation (CTBTO), which operates a global monitoring system for nuclear tests, said it did not detect any event consistent with a nuclear explosion on the date cited by the US. CTBTO Executive Secretary Robert Floyd said the organization’s monitoring system can reliably detect explosions equivalent to 500 tons of TNT or more, and no such signal was recorded.

Experts noted that extremely low-yield tests could potentially evade detection, though verification mechanisms under the Comprehensive Nuclear-Test-Ban Treaty (CTBT) cannot be activated because the treaty has not entered into force. While most countries have ratified the CTBT, the United States and China have not, and Russia withdrew its ratification in 2023.

China’s embassy in Washington rejected the accusation, saying Beijing adheres to a nuclear testing moratorium and follows a policy of “no first use” of nuclear weapons. A spokesperson said China remains committed to safeguarding the global nuclear non-proliferation regime and urged the US to uphold its own obligations.

DiNanno described the expiration of New START as “the end of an era,” adding that the US would proceed with nuclear modernization and retain the capacity to expand its arsenal if directed by President Donald Trump. US officials have argued that any future arms control framework must include China, a position Beijing has repeatedly resisted, citing its much smaller nuclear stockpile.