The U.S. government is claiming that China has conducted a secret nuclear test at its Lop Nur test site in 2020.

Thomas DiNanno, undersecretary of state for arms control and international security, recently said that the U.S has evidence of the test. 

“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,” DiNanno said during remarks at the United Nations Conference on Disarmament held on February 6. 

“China conducted one such yield-producing nuclear test on June 22 of 2020,” he added. 

The U.S. government provided further details about the alleged tests on Tuesday. 

According to Christopher Yeaw, the assistant secretary for arms control and nonproliferation at the State Department, a remote seismic station in Kazakhstan detected a minor earthquake on June 22, 2020. Although it registered only a 2.75 magnitude, the tremor originated roughly 450 miles away at China’s main nuclear test site, Lop Nur.

“There is very little possibility that it is anything other than an explosion, a singular explosion,” Yeaw said Tuesday at an event hosted by the Hudson Institute, a U.S. think tank. “It is quite consistent with what you would expect from a nuclear explosive test.”

Yeaw said that Washington is preparing its own low-yield nuclear tests in response to similar activities by China and Russia. This development follows the expiration of the New START treaty, a U.S.-Russia nuclear arms reduction agreement that limits the number of deployed strategic warheads.

Yeaw added that President Donald Trump was serious when he said in October that the U.S. would resume nuclear testing. He did not specify a timeline, but added that any test would be on a “level playing field.”

Meanwhile, China’s foreign ministry has denied the U.S. allegations of nuclear testing, saying that Beijing continues to uphold a voluntary moratorium on nuclear tests.