The United States accused Beijing on Friday of conducting a secret nuclear test in 2020 as it called for a new, broader arms control treaty that would bring in China as well as Russia.
The accusations at a global disarmament conference highlighted serious tension between Washington and Beijing at a pivotal moment in nuclear arms control, a day after the treaty limiting US and Russian missile and warhead deployments 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 tons," US Under Secretary of State for Arms Control and International Security Thomas DiNanno told a Disarmament Conference in Geneva.
The Chinese military "sought to conceal testing by obfuscating the nuclear explosions because it recognized these tests violate test ban commitments. China has used 'decoupling', a method to decrease the effectiveness of seismic monitoring, to hide their activities from the world," he said.
DiNanno said China had conducted one such "yield-producing test" on June 22, 2020.
China's ambassador on disarmament, Shen Jian, did not directly address DiNanno's charge but said Beijing had always acted prudently and responsibly on nuclear issues.
"China notes that the US continues in its statement to hype up the so-called China nuclear threat. China firmly opposes such false narratives," he said.
"It (the US) is the culprit for the aggravation of the arms race."
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.
GLOBAL ARMS CONTROL FACES A CRITICAL MOMENT
The 2010 New START treaty which ran out on Thursday left Russia and the United States for the first time since 1972 without any binding constraints on their deployments of strategic missiles and warheads.
US President Donald Trump wants to replace it with a new agreement including China, which is rapidly increasing its own arsenal. In the meantime, Washington says it will keep modernizing its own nuclear forces.
"Russia and China should not expect the United States to stand still while they shirk their obligations and expand their nuclear forces. We will maintain a robust, credible, and modernized nuclear deterrent," US Secretary of State Marco Rubio wrote in a post on the online publishing platform Substack.
DiNanno told the Geneva conference: "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."
He reiterated US projections that China will have over 1,000 nuclear warheads by 2030.
Shen, the Chinese delegate, reiterated that his country would not participate in new negotiations at this stage with Moscow and Washington. Beijing has previously highlighted that it has a fraction of their warhead numbers - an estimated 600, compared to around 4,000 each for Russia and the US.
"In this new era we hope the US will abandon Cold War thinking... and embrace common and cooperative security," Shen said.
Tomas Nagy, a nuclear expert at security think-tank GLOBSEC in Bratislava, said Washington had chosen this moment to call out Beijing for alleged secret testing from nearly six years ago because it felt Beijing was unlikely to cooperate on the issue.
"This is a reflection of the fact that the Americans have actually understood by now that for the next couple of years, there's going to be no motion in a positive direction with the Chinese. So they decided to disclose this information," he said in a phone interview.
Trump held what he called "very positive" talks with China's President Xi Jinping on trade and wider security issues this week and is due to visit Beijing in April.
EXPIRY OF NEW START LEAVES ARMS CONTROL VOID
Security analysts say a new nuclear arms control deal would take years to negotiate, with Russia and the US developing new weapons and tension over Ukraine, the Middle East and other flashpoints resulting in a higher risk of miscalculation.
Forced to rely on worst-case assumptions about the other's intentions, the US and Russia would see an incentive to increase their arsenals, especially as China plays catch-up.
Russia would prefer to have a dialogue with the United States after New START but is ready for any scenario, Foreign Minister Sergei Lavrov said on Friday. The Kremlin said the two sides, at talks in Abu Dhabi this week, had reached an understanding they would both act responsibly.
Russia says the nuclear allies of NATO members Britain and France should also be up for negotiation - something those countries reject.
At the Geneva forum, Britain said China, Russia and the US should come to an understanding, adding that it shared US concerns about Beijing's rapid expansion of its nuclear arsenal. France said agreement between states with the biggest nuclear arsenals was crucial at a time of an unprecedented weakening of nuclear norms.
