The U.S. State Department announced that China conducted a clandestine low-yield underground nuclear test.

“China’s possible preparation to operate its Lop Nur test site year-round, its use of explosive containment chambers, extensive excavation activities at Lop Nur and a lack of transparency on its nuclear testing activities … raise concerns regarding its adherence to the zero yield standard,” the state department report, first revealed by the Wall Street Journal, said.

The test would be in contravention of the Comprehensive Test Ban Treaty (CTBT) concluded in 1996 which included a mechanism for on-site inspections of suspect sites. The CTBT was signed by China and the U.S. Neither country ratified the treaty but China announced its intention to adhere to CTBT terms. The US has been observing a moratorium on nuclear testing. Russia, France and Britain, three of the world’s five internationally recognized nuclear powers, signed and ratified the CTBT but the treaty still requires ratification by 44 countries to become international law.

“Beijing is modernizing its nuclear arsenal while the United States handcuffs itself with one-sided arms-control,” Republican Senator Tom Cotton said on Twitter. “China has proven it can’t work with us honestly.”

The state department report claimed China blocked data transmissions from sensors linked to an international monitoring center. A spokeswoman for the Comprehensive Test Ban Treaty Organization (CTBTO), which verifies compliance with the pact, told the WSJ there had been no interruptions in data transmissions from China’s five sensor stations since September 2019. Before that, there were interruptions as a result of the negotiating process between the CTBTO and China on arrangements for putting the stations in operation.

Chinese foreign ministry spokesman Zhao Lijian discounted the claims.

“China has always adopted a responsible attitude, earnestly fulfilling the international obligations and promises it has assumed,” he said at a briefing in Beijing. “The US criticism of China is entirely groundless, without foundation, and not worth refuting.”