The US has conducted a non-nuclear underground experiment to enhance its ability to detect low-yield nuclear explosions that it explicitly linked with a test that Washington suspects China carried out in 2020 – an accusation Beijing denies.

The US Department of Energy announced on Wednesday that the National Nuclear Security Administration (NNSA) conducted a “subsurface, non-nuclear chemical high-explosive” experiment at the Nevada National Security Site, which it said aimed to “increase the United States’ ability to detect low-yield nuclear explosions”.

The experiment, which did not involve any nuclear explosions, collected data to validate scientific models and refine algorithms critical to US nuclear explosion detection capabilities.

The Nevada test highlights the constraints of verifying suspected low-yield nuclear tests. Despite Washington’s allegation that Beijing has covertly conducted a nuclear test at the Lop Nur testing site in June 2020, Beijing denied the accusation, and the international nuclear-test monitoring body has said the available seismic data cannot establish the cause of the events with confidence.

The US Energy Department’s press release said that the experiment would enhance understanding of the signature produced by underground detonation and strengthen the “unmatched” capability to detect any foreign nuclear testing.

The department said that the experiment was especially focused on further enhancing the US capabilities to detect a “decoupled” explosion – a nuclear test where the warhead is detonated inside a large underground cavity instead of being in close contact with the surrounding rock, lowering the seismic signal that the nuclear detection sensors can detect.

The Trump administration accused China of covertly carrying out a low-yield nuclear test in June 2020 at the Lop Nur nuclear test site in northwest China, alleging that China had breached the Comprehensive Nuclear-Test-Ban Treaty (CTBT).

US President Donald Trump has also ordered the resumption of US nuclear testing for the first time since 1992 on what he said was an “equal basis” in October 2025.

Beijing denies carrying out any covert nuclear testing Beijing denied the accusation and contended that it continues to abide by the terms of the CTBT. China’s last officially acknowledged nuclear test is known to have taken place at the Lop Nur site in July 1996, only two months before it signed the CTBT.

Both the US and China signed the CTBT in 1996, but neither country has ratified the treaty, so it has not yet entered into force. The South China Morning Post has asked the Chinese embassy in Washington for comment.

Daryl Kimball, executive director of the Arms Control Association, said that the NNSA experiment can “certainly enhance” US capabilities to detect “illicit” nuclear explosions that violate the CTBT, but detecting and actually deterring nuclear tests requires international cooperation.

He said that while the current Comprehensive Nuclear-Test-Ban Treaty Organisation’s (CTBTO) International Monitoring System provides powerful nuclear test monitoring capabilities, very low-yield nuclear explosions may still escape detection.

“Indeed, the US has accused China of conducting a low-yield nuclear test in June 2020, but the available data from the CTBTO’s monitoring system and other civilian seismic networks about the seismic event on that date are inconclusive,” said Kimball.

“This is why it is so important for nuclear-armed states to engage in technical discussions to develop voluntary confidence-building measures to ensure ongoing and future activities at their former test sites are fully compliant with their obligations under the CTBT.”The CTBTO has stated that its International Monitoring System did not detect any event consistent with a nuclear test explosion on June 22, 2020 – the date that Washington suspects China of conducting the test – though it also said that there were “two very small seismic events” spaced 12 seconds apart on that day.

Washington has ‘genuine concerns’ Russia, China conducted nuclear tests Tong Zhao, senior fellow at the Carnegie Endowment for International Peace, said that Washington has “genuine concerns” that Russia and China have conducted nuclear tests in violation of their commitments as signatories of the CTBT.

He said that the International Monitoring System established under the CTBT “may not reliably detect and distinguish very small nuclear tests”, where the NNSA experiment aims to enhance Washington’s own ability to “detect covert foreign nuclear tests”.

“Such a capability could strengthen deterrence by increasing concern among rival countries that prohibited nuclear testing could be detected despite efforts to hide it. It could also enable Washington to develop a better understanding of foreign nuclear-warhead modernisation programmes,” said Zhao.

“If China were indeed seeking to advance its nuclear-warhead designs through nuclear testing inconsistent with its CTBT commitment, improvements in US detection capabilities could complicate such efforts.”China has recently accelerated its nuclear stockpile. The Pentagon’s China military power report published in December last year estimated that the People’s Liberation Army remains on track to have over 1,000 warheads by 2030.

According to a report by the Stockholm International Peace Research Institute (SIPRI), China is estimated to have a nuclear stockpile of up to 620 warheads as of January 2026 and is expected to “keep growing over the coming decade”.

Robert Kelley, a former official at the US Energy Department and the director of nuclear inspections at the International Atomic Energy Agency (IAEA), said that the focus of the experiment seems to be to detect very small nuclear tests that could be used to collect data about “how a larger bomb may work” or to examine a new technology aimed at disruption of electronic systems.

He said the experiment would help the US understand Chinese testing, and that the tests are part of a longer effort to detect “smaller and smaller underground tests since at least 1963”.

“China might be doing very advanced nuclear experiments, even what are called ‘subcritical tests’ to polish very high-end complex physics knowledge,” said Kelley, who now serves as the Distinguished Associate Fellow at SIPRI.

John Erath, senior policy director at the Centre for Arms Control and Non-Proliferation, said that the test was part of “long-standing efforts” by the US to identify any nuclear explosion.

“Such efforts are designed to detect nuclear tests, including those that would be contrary to the CTBT,” said Erath.

“The Nevada experiment is specifically designed to improve detection of decoupled nuclear explosions, such as allegedly occurred at Lop Nur in 2020. This would likely be a reason for linking the current [US Energy Department] experiment to activity at Lop Nur.”