Washington wants Beijing to join a new nuclear weapons treaty after expiration of the New START accord between the US and Russia.

An international monitor said it has seen no evidence to support the claim by a senior United States official who accused China of carrying out a series of clandestine nuclear tests in 2020 and concealing activities that violated nuclear test ban treaties.

US Under Secretary of State for Arms Control and International Security Thomas DiNanno made the assertions about China at a United Nations disarmament conference in Geneva, Switzerland, on Friday, just days after a nuclear treaty with Russia expired.

“I can reveal that the US government is aware that China has conducted nuclear explosive tests, including preparing for tests with designated yields in the hundreds of tonnes,” DiNanno said at the conference.

China’s military “sought to conceal testing by obfuscating the nuclear explosions because it recognised these tests violate test ban commitments,” he said.

“China conducted one such yield-producing nuclear test on June 22 of 2020,” he said.

DiNanno also made his allegations on social media in a series of posts, making the case for “new architecture” in nuclear weapons control agreements following the expiration of the New START treaty with Russia this week.

“New START was signed in 2010 and its limits on warheads and launchers are no longer relevant in 2026 when one nuclear power is expanding its arsenal at a scale and pace not seen in over half a century and another continues to maintain and develop a vast range of nuclear systems unconstrained by New START’s terms,” he said.

Robert Floyd, executive secretary of the Comprehensive Nuclear-Test-Ban Treaty Organization, said in a statement on Friday that the body’s monitoring system “did not detect any event consistent with the characteristics of a nuclear weapon test explosion” at the time of the alleged Chinese test, adding that that assessment remains unchanged after further detailed analyses.

China’s ambassador on nuclear disarmament, Shen Jian, did not directly address DiNanno’s charge at the conference but said Beijing had always acted prudently and responsibly on nuclear issues while the US had “continued to distort and smear China’s national defence capabilities in its statements”.

“We firmly oppose this false narrative and reject the US’s unfounded accusations,” Shen said.

“In fact, the US’s series of negative actions in the field of nuclear arms control are the biggest source of risk to international security,” he said.

Later on social media, Shen said, “China has always honored its commitment to the moratorium on nuclear testing”.

Diplomats at the conference said the US allegations were new and concerning.

China, like the US, has signed but not ratified the Comprehensive Nuclear Test Ban Treaty (CTBT), which bans explosive nuclear tests. Russia signed and ratified it, but withdrew its ratification in ⁠2023.

US President Donald Trump has previously instructed the US military to prepare for the resumption of nuclear tests, stating that other countries are conducting them without offering details.

The US president said on October 31 that Washington would start testing nuclear weapons “on an equal basis” with Moscow and Beijing, but without elaborating or explaining what kind of nuclear testing he wanted to resume.

He has also said that he would like China to be involved in any future nuclear treaty, but authorities in Beijing have shown little interest in his proposal.