The United States has revealed new details regarding an alleged underground nuclear test conducted by China in June 2020.

Axar.az, citing Reuters, reports that according to Assistant Secretary of State Christopher Yeaw, seismic data from a monitoring station in Kazakhstan detected a magnitude 2.75 “explosion” near China’s Lop Nor test site.

Yeaw stated that the seismic signature was inconsistent with mining activity or an earthquake and matched characteristics expected from a nuclear explosive test.

However, the Comprehensive Test Ban Treaty Organization (CTBTO), which oversees global nuclear test monitoring, said there is insufficient evidence to confirm the event as a nuclear explosion. The station involved (PS23) reportedly detected two small seismic events 12 seconds apart.

China has firmly denied the allegation. A spokesperson for the Chinese embassy in Washington described the U.S. claim as “unfounded” and accused Washington of political manipulation. Beijing maintains that it complies with its obligations under the 1996 nuclear test ban treaty, which it has signed but not ratified. China’s last officially acknowledged nuclear test occurred in 1996.

The issue arises amid growing global nuclear tensions following the expiration of New START earlier this month. U.S. President Donald Trump is urging China to join future arms-control negotiations with Washington and Moscow, raising concerns about a potential new nuclear arms race.