The United States on Friday accused China of carrying out a secret nuclear test in 2020 as the Trump administration calls for a broader nuclear weapons agreement including both China and Russia.
The allegation comes a day after thelast remaining nuclear arms control treatybetween the US and Russia lapsed, leaving the world's largest nuclear superpowers without limits on their arsenals for the first time in decades.
President Donald Trump and other top officials in his administration have made clear they will no longer abide by the limitations of the New START Treaty and instead have argued they need a new deal to address threats from Moscow and Beijing. And Trump last year called for the resumption of US nuclear weapons tests.
"Today, 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," Undersecretary of State for Arms Control and International Security Thomas DiNanno said in remarks at a global Disarmament Conference in Vienna Friday.
"China conducted one such yield producing nuclear test on June 22 of 2020," he said, without providing further details. A former senior US official told CNN that information about China's 2020 test had been declassified.
DiNanno accused the Chinese military of seeking "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. According to experts, decoupling happens when a large cavern is dug to lessen the seismic activity from a nuclear explosion, making it harder to detect.
A top official from an organization that works to monitor for nuclear weapons tests worldwide said in a statement Friday that their system "did not detect any event consistent with the characteristics of a nuclear weapon test explosion" on June 22, 2020.
"Subsequent, more detailed analyses have not altered that determination," said Rob Floyd, the executive secretary of the Comprehensive Nuclear-Test-Ban Treaty Organization (CTBTO).
Floyd said the organization's International Monitoring System (IMS) "is capable of detecting nuclear test explosions with a yield equivalent to or greater than approximately 500 tonnes of TNT." He noted it had detected "all six tests conducted and declared by" North Korea.
The alleged Chinese test had a yield "in the hundreds of tons," DiNanno said without providing a specific number, so it's unclear if it would have met the threshold to be be detected the monitoring system.
"If this was a very, very low yield test explosion…it is possible that it could be hidden from the CBTBO monitoring stations," explained Daryl Kimball, the Executive Director of the Arms Control Association.
Floyd noted that there are mechanisms "which could address smaller explosions" provided by the Comprehensive Test Ban Treaty (CTBT). That treaty prohibits "any nuclear weapon test explosion or any other nuclear explosion." However, those mechanisms can only be used when the treaty enters into force.
Most of the world has signed onto and ratified the treaty. Although both the US and China signed, they have not ratified it and Russia withdrew its ratification in 2023. As such, the Treaty cannot enter into force.
The US and China in the past had said they adhere to a moratorium on nuclear testing, but last year, Trump called for US nuclear weapons testing to resume "on an equal basis."
In his remarks on Friday, DiNanno suggested that the alleged Chinese testing had motivated Trump's decree. He also said that "the annual US compliance report has previously assessed that Russia has failed to maintain its testing moratorium by conducting supercritical nuclear weapons tests."
Asked about the allegation of secret nuclear testing, the spokesperson for the Chinese Embassy in Washington, DC, said China "follows a policy of 'no first use' of nuclear weapons and a nuclear strategy that focuses on self-defense, and adheres to its nuclear testing moratorium."
"We stand ready to work with all parties to jointly uphold the authority of the Comprehensive Nuclear Test-Ban Treaty and safeguard the international nuclear disarmament and non-proliferation regime," said Liu Pengyu.
"It's hoped that the US will earnestly abide by its obligations under the Treaty and its commitment to a moratorium on nuclear testing and take concrete actions to uphold the international nuclear disarmament and non-proliferation regime, as well as global strategic balance and stability," he told CNN.
'The end of an era'
In his remarks Friday, DiNanno said that "February 5, 2026, indeed marks the end of an era: the end of US unilateral restraint," referencing the end of the New START Treaty. Although he did not explicitly say the US would upload additional nuclear weapons now that it was no longer bound by the agreement, he indicated it was likely.
"We will complete our ongoing nuclear modernization programs that were initiated while New START entered into force. The United States also retains non-deployed nuclear capacity that can be used to address the emerging security environment, if directed by the president," he said.
The US "will maintain a robust, credible, and modernized nuclear deterrent to ensure our security preserves peace and stability, and negotiate from a position of strength," he added.
"The next era of arms control can and should continue with clear focus, but it will require the participation of more than just Russia at the negotiating table," DiNanno said.
It is unclear how the US intends to get China to that negotiating table. Beijing has consistently rebuffed trilateral arms control negotiations, arguing that their stockpiles are not on par with Moscow and Washington.
Matthew Kroenig, vice president and senior director of the Atlantic Council's Scowcroft Center for Strategy and Security, noted that if this is truly China's concern, "shouldn't they want arms control?"
"If they can get us to limit our weapons, that should be better for them," he argued. He said he believes Beijing doesn't want to negotiate because "they want a superpower nuclear force."
"They've invested a lot in building this force. They didn't spend all this money and bend all this metal to trade it away," he said.
Some US officials believe that the expiration of New START paves the way for the expansion of the US arsenal which could prompt enough Chinese concern to bring the expanding nuclear power the table, according to a US official.
Daryl Kimball, the Executive Director of the Arms Control Association, noted that "if there is any true violation of the test ban treaty, that's a big problem, but simply complaining about it doesn't solve the problem."
He called on the US to propose a "sensible approach" like bilateral talks over arms control.
"In the meantime, there is no reason why the United States and Russia should not and cannot continue to respect the central limits of New START," he said.
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