The United States on Friday accused China of conducting a secret nuclear test in 2020, as the Trump administration seeks a broad nuclear weapons deal that includes both China and Russia.

The allegations come a day after the last remaining nuclear weapons control treaty between the United States and Russia expired, leaving the world’s largest nuclear superpower with no limits on its arsenal for the first time in decades.

President Donald Trump and other senior officials in his administration have vowed to no longer abide by the New START treaty’s restrictions, insisting instead that a new agreement is needed to address threats from Russia and China. And last year, President Trump called for the U.S. to resume nuclear weapons testing.

“Today, I can make clear that the U.S. government is aware that China has conducted nuclear explosion tests, including preparations for nuclear explosion tests with designated yields of several hundred tons,” Thomas DiNanno, Under Secretary of State for Arms Control and International Security, said Friday in remarks at the World Conference on Disarmament in Vienna.

“China conducted one nuclear test with such a yield on June 22, 2020,” he said, without providing further details. A former senior U.S. official told CNN that information about China’s 2020 nuclear test has been declassified.

DiNanno accused the Chinese military of trying to “cover up the test by obscuring the nuclear explosion because it recognized that it violated the promise of a non-test.”

“China has used decoupling (a technique that reduces the effectiveness of earthquake monitoring) to hide its activities from the world,” he said. Experts say decoupling occurs when large cavities are dug to reduce seismic activity from nuclear explosions, making them difficult to detect.

Executives at the organization, which works to monitor nuclear weapons tests around the world, said in a statement Friday that its systems “did not detect any events consistent with the characteristics of a nuclear weapons test explosion” on June 22, 2020.

Rob Floyd, executive director of the Comprehensive Nuclear-Test Ban Treaty Organization (CTBTO), said: “Further detailed analysis has not changed that decision.”

Floyd said the organization’s International Monitoring System (IMS) “can detect nuclear test explosions equal to or greater than approximately 500 tons of TNT.” He noted that North Korea had detected “all six tests that it conducted and declared.”

DiNanno said tests attributed to China showed a yield of “several hundred tons,” without giving a specific number, so it was unclear whether it met the criteria for detection by surveillance systems.

“If this was a very low-yield experimental explosion…it could be hidden from the CBTBO monitoring station,” explained Darryl Kimball, executive director of the Arms Control Association.

Floyd noted that there are mechanisms provided by the Comprehensive Nuclear-Test-Ban Treaty (CTBT) that “can deal with small explosions.” The treaty prohibits “test explosions of nuclear weapons and other nuclear explosions.” However, these mechanisms can only be used once the treaty enters into force.

Most countries in the world have signed and ratified this convention. The treaty cannot enter into force because the United States and China have signed but not ratified it, and Russia withdrew its ratification in 2023.

The United States and China have said in the past that they would stick to a moratorium on nuclear testing, but President Trump last year called for U.S. nuclear weapons testing to resume “on an equal basis.”

In his remarks Friday, DiNanno suggested that China’s alleged nuclear test was a motivator for President Trump’s executive order. He also said that “the United States’ annual compliance report previously assessed Russia’s failure to maintain a moratorium on nuclear testing by conducting supercritical nuclear weapons tests.”

Asked about the suspicions of secret nuclear tests, a spokesperson for the Chinese embassy in Washington, D.C., said, “China firmly adheres to the moratorium on nuclear tests, in accordance with its ‘no first use’ policy of nuclear weapons and its nuclear strategy that emphasizes self-defense.”

“We stand ready to cooperate 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,” Liu Pengyu said.

“We hope that the United States will earnestly honor its treaty obligations and its commitment to suspend nuclear testing, and take concrete actions to maintain the international nuclear disarmament and non-proliferation regime and the world’s strategic balance and stability,” he told CNN.

“February 5, 2026 certainly marks the end of an era. It is the end of unilateral U.S. restraint,” DiNanno said in Friday’s remarks, referring to the end of the New START treaty. Although he did not say explicitly that the United States would upload additional nuclear weapons now that it is no longer bound by the deal, he suggested that was likely.

“We will complete ongoing nuclear modernization programs begun while New START took effect. The United States also maintains undeployed nuclear capabilities that can be used to address emerging security environments at the direction of the President.”

The United States “will maintain a strong, reliable, modernized nuclear deterrent and negotiate from a position of strength to ensure our national security maintains peace and stability,” he added.

“The next era of arms control can and should continue with a clear focus, but it will require more than just Russia at the negotiating table,” DiNanno said.

It is unclear how the United States intends to bring China to the negotiating table. Beijing has consistently rejected trilateral arms control negotiations, arguing that its stockpiles are not on par with those of Russia or the United States.

Matthew Kroenig, vice president and senior director of the Atlantic Council’s Scowcroft Center for Strategic Security, said if this is indeed a concern for China, “shouldn’t we want arms control?”

“If they can get us to restrict weapons, that should be better for them,” he argued. He said he believed Beijing did not want to negotiate because it “desires the nuclear arsenal of a superpower.”

“They’ve invested a lot in building this unit. They’ve spent all this money and they haven’t bent all this metal and replaced it,” he said.

Some U.S. officials believe the expiration of New START would pave the way for a U.S. military expansion and could raise enough concerns in China to bring expanding nuclear power to the negotiating table, according to some U.S. officials.

“If there is an actual violation of the Nuclear-Test Ban Treaty, that’s a big problem, but just complaining about it won’t solve the problem,” said Darryl Kimball, executive director of the Arms Control Association.

He called on the US to propose “sensible approaches” such as bilateral talks on arms control.

“In the meantime, there is no reason why the United States and Russia should not and cannot continue to respect the core limits of New START,” he said.