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A top Trump administration official on Tuesday disclosed new details to support its claim that China conducted an underground nuclear explosion during Donald Trump's first presidency, a contested assertion that, nevertheless, has become a catalyst for his push to resume such testing by the United States.
"We're not going to remain at an intolerable disadvantage," Christopher Yeaw, who heads the State Department's Bureau of Arms Control and Nonproliferation, said during a discussion at the Hudson Institute, a think tank in Washington. He urged Beijing to "come clean" about its nuclear weapons tests, which some U.S. officials and experts say is part of a determined Chinese effort to catch up to, if not eclipse, the United States' global advantage in nuclear weapons technology.
The event in question occurred June 22, 2020, "right near" a secretive facility known as Lop Nur in China's western Xinjiang region, Yeaw told those attending the discussion. The U.S., he added, used seismic data captured by a facility in neighboring Kazakhstan to conclude that China had conducted an explosive nuclear test.
The activity - measured at a magnitude of 2.76 - was not consistent with an earthquake or explosions used in mining, Yeaw said. The suspected explosion's yield - meaning the amount of energy it released - remains unclear because of the Chinese government's efforts to conceal the test, he said, adding, "We know that they were preparing designated tests of hundreds of tons" in yield.
Yeaw's remarks appeared intended to dispel skepticism of the Trump administration's allegation earlier this month that China had conducted a secret explosive test nearly six years ago. Independent experts have said that seismic data, even if combined with satellite data, would probably prove inconclusive.
In a statement Tuesday, the primary international body that uses seismic sensors to detect nuclear explosions, the Comprehensive Nuclear-Test-Ban Treaty Organization, said it detected "two very small seismic events, 12 seconds apart" in the time period Yeaw described, but that they were too small to "assess the cause of these events with confidence."
The Chinese Embassy in Washington did not immediately respond to a request for comment Tuesday. Earlier this month, a spokesman there said "China is a responsible nuclear state" and that it adheres to a nuclear testing moratorium.
The administration's focus on the event follows a claim, made by Trump in October, that both China and Russia had undertaken nuclear explosive testing. He has pledged to resume U.S. nuclear explosive testing "on an equal basis" to those nations.
Trump's announcement alarmed arms control experts, as any resumption in such testing would reverse a post-Cold War taboo in the U.S. that has held since the last nuclear weapon test was conducted in 1992.
After the last remaining nuclear arms limitation treaty between the United States and Russia lapsed Feb. 5, Trump wrote on social media that he hoped to replace it with a "new, improved, and modernized" agreement that would include not only the United States and Russia, the two largest nuclear arsenals in the world, but also China, a fast-growing nuclear power, too.
Analysts say that publicly detailing a Chinese test could be a pressure tactic aimed at pushing Beijing to become involved in such talks.
China has long declined to participate in agreements like the one that lapsed this month, called New START, arguing its arsenal is far smaller than those of Russia and the United States. Trump and other U.S. officials have said that while China's stockpile is smaller, it is expanding rapidly - and that Beijing is not bound by the same testing constraints that Washington has pledged to abide by.
China has about 600 nuclear warheads, according to a Pentagon report released in December. By comparison, Russia possesses roughly 4,300 warheads and the United States about 3,700 as of January 2025, according to the Stockholm International Peace Research Institute's latest annual assessment.
The United States, Russia and China are signatories to 1996 Comprehensive Nuclear-Test-Ban Treaty, which prohibits "any nuclear weapon test explosion or any other nuclear explosion," though the United States and China never ratified the treaty, and Russia rescinded its ratification in 2023.
The last confirmed nuclear explosive test in Russia was conducted during the dying days of the Soviet Union in 1990. China's last occurred at Lop Nur in 1996. The State Department and the Defense Department have in recent years suggested that both countries have conducted nondisclosed tests.
Thomas DiNanno, the undersecretary of state for arms control, was the first U.S. official to allege that China had conducted a nuclear explosive test in 2020. He said that China's military had "sought to conceal" its efforts because "it recognized these tests violate test ban commitments."
Yeaw said that it was "impossible" to tell from the seismic data he discussed Tuesday how big the explosion was in 2020. China, he alleged, was using "decoupling" techniques - such as detonating devices deep underground - to muffle the blast and confuse international monitoring systems.
Still, he said, it was "pretty obvious" that the explosion was at least "supercritical," a type of test that uses a limited amount of nuclear material but does not produce a chain reaction.
Tracking China's nuclear development - including its testing capabilities - is extraordinarily difficult, and few outside observers have ever set foot inside a Chinese military nuclear facility.
Analysts say that while it is unlikely U.S. authorities could make a conclusive determination based on seismic data alone, they cannot rule out the possibility that the Trump administration's claims are supported by other intelligence - including classified data that can detect radioactive signatures or human intelligence.
If such evidence does exist, U.S. officials have not disclosed it.
(COMMENT, BELOW)