A senior official in President Donald Trump’s administration provided new details supporting Washington’s claim that China conducted an underground nuclear test during Trump’s first term.

Christopher Yoo, head of the State Department’s Bureau of Arms Control and Nonproliferation, said at an event at the Hudson Institute in Washington that the United States would not remain in an “unacceptable position” in terms of strategic advantage and urged Beijing to be “fully transparent” about its nuclear testing activities.

He said some U.S. officials and experts believe the activities are part of China’s accelerated efforts to catch up with or even surpass the United States in nuclear weapons technology.

Yoo said the event in question occurred on June 22, 2020, near a secret facility known as Lop Nur in China’s western Xinjiang region.

China’s foreign ministry did not immediately comment on the remarks.