"Today, I can reveal that the US government is aware that China has conducted nuclear explosive tests, including preparing for tests with designated yields in the hundreds of tonnes," Thomas DiNanno, US under secretary of state for arms control, told the UN Conference on Disarmament in Geneva.

"China conducted one such yield-producing nuclear test on June 22 of 2020," he said.

US President Donald Trump had already hinted at similar accusations late last year, but without providing the same level of detail.

Trump said on October 31 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 on Friday maintained that the People's Liberation Army in China had "sought to conceal testing by obfuscating the nuclear explosions because it recognised these tests violate test ban commitments".

He accused China of using "decoupling -- a method to decrease the effectiveness of seismic monitoring -- to hide their activities from the world".

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 on Thursday.

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