Washington said the lapsing earlier this month of New START -- the last treaty between top nuclear powers the United States and Russia -- presented the possibility to achieve a "better agreement", including Beijing.

China has publicly rejected calls to enter negotiations on a new three-way treaty.

Christopher Yeaw, the US assistant secretary of state for arms control and nonproliferation, told the Conference on Disarmament in Geneva that the New START treaty had been seriously flawed.

"Perhaps its greatest flaw was that New Start did not account for the unprecedented, deliberate, rapid and opaque nuclear weapons build-up by China," he said.

"Despite its claims to the contrary, China has deliberately and without constraint, massively expanded its nuclear arsenal without transparency or any indication of China's intent or end point," he charged.

"We believe China may achieve parity within the next four or five years," he said, without elaborating what he meant by parity.

Both Russia and the United States have more than 5,000 nuclear weapons, according to the Nobel Peace Prize-winning campaign group ICAN.

But New START, which expired on February 5, restricted the United States and Russia to 1,550 deployed nuclear warheads each -- a number Washington says China is fast approaching.

"Beijing is on track to have the fissile material necessary for more than 1,000 nuclear warheads by 2030," Yeaw said.

The expiration of New START marks the first time in decades that there is no treaty to curtail the positioning of the planet's most destructive weapons, sparking fears of a fresh arms race.

Yeaw welcomed the lapsing of the treaty, insisting its numerical limits on warheads and launchers were "no longer relevant", given Russia's alleged violation of the treaty.

He also accused Moscow of helping "boost Beijing's capacity to increase its arsenal size".

"The expiration arrived at a fortuitous time", he said, insisting it would allow US President Donald Trump to push towards his "ultimate goal of a better agreement".

"The treaty's expiration and the absence of any nuclear arms control treaty right now does not mean the United States is walking away from or ignoring arms control," he said, insisting: "Quite the opposite is true."

"Our goal, is a better agreement toward a world with fewer nuclear weapons."