US accuses China of breaking rules on nuclear weapons by hiding explosions and preparing for more testing

• The US said it has intelligence that China conducted banned nuclear explosive tests.
• The nuclear monitoring organization said it hadn't detected activity related to an explosive test.
• China is expanding its nuclear arsenal. An explosive test would violate an international agreement.

The US accused China of secret nuclear explosive testing and attempting to cover it up, an official said Friday.

Banned for decades under an international agreement to which China and the US are signatories, nuclear explosive testing involves the detonation of warheads. The US accusation comes on the heels of the expiration of the nuclear arms control New START agreement between the US and Russia, which limited their numbers of deployed warheads and launchers.

"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 said in Geneva, Switzerland.

DiNanno added that Beijing had used "decoupling — a method to decrease the effectiveness of seismic monitoring — to hide its activities from the world," and said one such test had occurred on June 22, 2020. Decoupling refers to muffling the seismic signal of an underground nuclear test.

A spokesperson for China's embassy in the US told Business Insider that Beijing is "committed to peaceful development, 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." The spokesperson added China is ready to work with other countries to uphold the agreement.

The Comprehensive Nuclear-Test-Ban Treaty, adopted in 1996, banned the live tests that have occurred for decades and contributed to nuclear scares and radioactive exposure. The last such test conducted by the US was in 1992, and China's most recent occurred in 1996. The only publicly known explosive testing this century has been by North Korea. The US, Russia, and China have signed the treaty but haven't ratified it, unlike nearly 180 other countries.

Current testing practices rely on modeling, simulation, and subcritical testing, which officials have said are vigorous and dependable. Last year, US President Donald Trump raised concerns with vague comments on resuming nuclear testing in response to other countries.

"They seem to all be nuclear testing. We have more nuclear weapons than anybody. We don't do testing, and we've halted it years, many years ago. But with others doing testing, I think it's appropriate that we do also," the president said.

There's been some suspicion that US rivals have been preparing for testing, including work spotted at the Lop Nur and Novaya Zemlya nuclear sites in China and Russia, respectively.

But in response to DiNanno's comments, Robert Floyd, executive secretary of the CTBTO, said the organization's monitoring system "did not detect any event consistent with the characteristics of a nuclear weapon test explosion at that time. Subsequent, more detailed analyses has not altered that determination."

Just a day before DiNanno's comments, the New START agreement binding the US and Russia to a number of operational nuclear weapons and launchers, as well as a verification process and open communication, expired. Whether Washington and Beijing will continue to observe the treaty or attempt a new agreement remains unclear.

The Trump administration has said a new nuclear arms deal would need to account for the growth of China's nuclear weapons stockpile, which the Pentagon has assessed will top 1,000 by 2030. "And as we sit here today, China's entire nuclear arsenal has no limits, no transparency, no declarations, and no controls," DiNanno said, adding that a post-New START treaty will address both Russia's vast arsenal and China's nuclear rise.