At the UN Conference on Disarmament in Geneva on Friday, Thomas DiNanno, US under secretary of state for arms control accused China of conducting the tests, including one on 22 June 2020, and of preparing for more tests with massive yields.

"The US allegations are completely groundless and are outright lies. China firmly opposes the US attempt to fabricate excuses for its own restarting of nuclear tests," China's ministry of foreign affairs said in a statement to AFP on Monday.

It also urged the United States to "immediately stop its irresponsible actions".

US President Donald Trump said in October that Washington would start testing nuclear weapons "on an equal basis" with Moscow and Beijing, but without elaborating or explaining what kind of nuclear testing he wanted to resume.

DiNanno's comments came as he was presenting a new US plan calling for three-way talks with Russia and China to set new limits on nuclear weapons, after the expiration of New START -- the last treaty between top nuclear powers Washington and Moscow, which expired last Thursday.

China has already rejected joining disarmament negotiations "at this stage".