The United States has accused Beijing of conducting a covert nuclear test in 2020, adding fresh strain to already fraught relations as Washington presses for a broader arms control treaty to include China as well as Russia.

The allegations, delivered on Friday at a global disarmament conference, underscored rising tensions between Washington and Beijing at a pivotal moment in nuclear diplomacy, just a day after the treaty limiting U.S. and Russian missile and warhead deployments expired.

“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,” U.S. Under Secretary of State for Arms Control and International Security Thomas DiNanno told a Disarmament Conference in Geneva.

He said the Chinese military “sought to conceal testing by obfuscating the nuclear explosions because it recognised these tests violate test ban commitments. China has used ‘decoupling’, a method to decrease the effectiveness of seismic monitoring, to hide its activities from the world.”

DiNanno said China had conducted one such "yield-producing test" on 22 June, 2020.

• Why the New START nuclear treaty matters

Beijing rejects the allegation

China’s ambassador on disarmament, Shen Jian, did not directly address the testing charge but said Beijing had always acted prudently and responsibly on nuclear matters.

“China notes that the U.S. continues in its statement to hype up the so-called China nuclear threat. China firmly opposes such false narratives,” he said. “It is the U.S. that is the culprit behind the aggravation of the arms race.”

Diplomats at the conference described the U.S. allegations as new and concerning.

Arms control enters a dangerous new era

New START expired on Thursday, leaving Russia and the United States without binding limits on their strategic arsenals for the first time in more than fifty years.

• Russia says New START obligations no longer apply

U.S. President Donald Trump wants a new agreement that brings in China, which Washington says is rapidly expanding its nuclear arsenal.

"Today, the United States faces threats from multiple nuclear powers. In short, a bilateral treaty with only one nuclear power is simply inappropriate in 2026 and going forward," DiNanno said.

He repeated U.S. projections that China will have more than 1,000 nuclear warheads by 2030. Beijing, however, insists its estimated 600 warheads remain a fraction of the Russian and U.S. stockpiles, each of which numbers around 4,000.

Shen reiterated that China would not join new negotiations at this stage. “In this new era we hope the U.S. will abandon Cold War thinking and embrace common and cooperative security,” he said.

A void after New START

The treaty’s expiry leaves a vacuum in arms control frameworks that have helped stabilise relations between Washington and Moscow since 1972.

Without replacement limits, analysts warn that both sides could revert to worst-case assumptions and expand their arsenals, especially as China accelerates its own capabilities.

Russia said it preferred renewed dialogue with the United States but was prepared for any scenario.

The Kremlin noted that both sides recognised the need to launch talks soon and that discussions in Abu Dhabi this week produced an understanding they would "act responsibly."

Moscow argues that NATO nuclear allies Britain and France must be part of any future agreement, a position both countries have rejected.

• Russia ready for world without nuclear arms limits after New START expiry, senior diplomat says

Rising global concern over nuclear risks

Britain told the Geneva forum it was time for a new era of arms control that included China, Russia and the United States.

France said an agreement among states with the largest arsenals was crucial amid an “unprecedented weakening of nuclear norms.”

Negotiating such deals has become increasingly complex. Russia is developing new systems, including the Burevestnik nuclear-powered cruise missile and the Poseidon underwater torpedo, while Trump has pledged to build a space-based “Golden Dome” missile defence.

• New START expiry risks unpredictability and loss of nuclear limits, analyst warns

Security analysts warn that any new framework may take years to negotiate, prolonging the current void at a time of heightened tensions involving Ukraine, the Middle East and other flashpoints.

Some analysts say this uncertainty could fuel debates in Japan, South Korea and Poland over whether they should seek their own nuclear capabilities.

Tags

START treaty China United States Nuclear Threat nuclear testing