The US state department has claimed China may have secretly conducted a low-yield underground nuclear test, in an accusation likely to further inflame already poor relations between Washington and Beijing.

A report on arms control compliance does not offer proof, but points to circumstantial evidence, of excavations and other stepped-up activity at China’s Lop Nur test site.

“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.

Both the US and China signed the Comprehensive Test Ban Treaty (CTBT), concluded in 1996, but neither country has ratified it, and – partly as a result – the agreement has not come into force. However, China has sworn to adhere to CTBT terms and the US has been observing a moratorium on nuclear testing.

If the treaty were in force, it would include a mechanism for on-site inspections of suspect sites.

The US defence intelligence agency leveled similar accusations against Russia in May last year, which were never confirmed. US hawks have been urging the Trump administration to formally break from the CTBT, leaving it free to conduct new nuclear tests of its own.

“Beijing is modernising 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.”

Jeffrey Lewis, a nuclear weapons expert at the Middlebury Institute of International Studies, said that the available evidence was possibly consistent with a low-yield tests or with “sub-critical tests”, which do not involve nuclear fission, and which are allowed by the CTBT.

“It is worth noting how thin the evidence is for these claims,” Lewis wrote. “US, Russia and China all conduct subcritical tests…From satellites and seismic stations, subcritical tests are indistinguishable from low yield nuclear tests.”

The finding may worsen ties already strained by US charges that the global Covid-19 pandemic resulted from Beijing’s mishandling of a 2019 outbreak of the coronavirus in the city of Wuhan.

The evidence cited by the state department report claimed Beijing’s included blocking data transmissions from sensors linked to an international monitoring center. However, a spokeswoman for the Comprehensive Test Ban Treaty Organization (CTBTO), which verifies compliance with the pact, told the Journal 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 told a daily briefing in Beijing that China was committed to a moratorium on nuclear tests and said the United States was making false accusations.

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

A senior US official said the concerns about China’s testing activities buttressed President Donald Trump’s case for getting China to join the US and Russia in talks on an arms control accord to replace the 2010 New Start treaty between Washington and Moscow that expires in February next year.

New Start restricted the US and Russia to deploying no more than 1,550 nuclear warheads, the lowest level in decades, and limited the land- and submarine-based missiles and bombers that deliver them.

“The pace and manner by which the Chinese government is modernising its stockpile is worrying, destabilising, and illustrates why China should be brought into the global arms control framework,” said the senior US official on condition of anonymity.

China, estimated to have about 300 nuclear weapons, has repeatedly rejected Trump’s proposal, arguing its nuclear force is defensive and poses no threat.

Russia, France and Britain – three of the world’s five internationally recognised nuclear powers – signed and ratified the Comprehensive Test Ban Treaty, which still requires ratification by 44 countries to become international law.