WASHINGTON, April 15 (Reuters) - China may have secretly set off low-level underground nuclear test explosions despite claiming to observe an international pact banning such blasts, the U.S. State Department said in a report on Wednesday that could fuel U.S.-Chinese tensions.

Reuters: U.S. concerns about Beijing's possible breaches of a "zero yield" standard for test blasts have been prompted by activities at China's Lop Nur nuclear test site throughout 2019, the State Department report said.

Reuters: Zero yield refers to a nuclear test in which there is no explosive chain reaction of the type ignited by the detonation of a nuclear warhead.

Reuters: “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 ...”

Reuters: “...raise concerns regarding its adherence to the zero yield standard," the report said, without providing evidence of a low-yield test.

Reuters: Beijing's lack of transparency included blocking data transmissions from sensors linked to a monitoring center operated by the international agency that verifies compliance with a treaty banning nuclear test explosions.

Reuters: A spokeswoman for the Comprehensive Test Ban Treaty Organization told WSJ there had been no interruptions in data transmissions from China's five sensor stations since the end of August 2019 following an interruption that began in 2018.

Reuters: A senior U.S. official said the concerns about China's testing activities buttressed President Donald Trump's case for getting China to join the United States and Russia in talks on an arms control accord to replace the 2010 New START treaty between Washington and Moscow

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