Washington said this week that China's nuclear arsenal was developing much faster than US projections had previously anticipated, and Beijing is likely to have more than 1,000 operational nuclear warheads by 2030.

Asked about the claim, China's foreign ministry expressed its "firm opposition" to the US report, though a spokesperson did not outright deny the numbers given.

"China firmly pursues a nuclear strategy of self-defence," foreign ministry spokesperson Mao Ning said.

"We have always kept our nuclear forces at the minimum level required for national security and have no intention to engage in a nuclear arms race with any country," she added.

"No country will be threatened by China's nuclear weapons as long as it does not use or threaten to use nuclear weapons against China," Mao said.

She also hit back at US moves to "invest heavily in upgrading its nuclear" forces and its policy of providing nuclear protection to non-nuclear allies, formally known as "extended deterrence".

"These policy actions aggravate the risk of a nuclear arms race and nuclear conflict, and will only worsen the global strategic security environment," Mao warned.

The United States currently possesses about 3,700 nuclear warheads, trailing Russia's roughly 4,500, according to the Stockholm International Peace Research Institute, which counts 410 warheads for China.

Beijing officially adopts a nuclear policy of "no first use" -- stating it will only use its nuclear weapons if attacked first.

But in recent years, under President Xi Jinping, it has begun a massive military modernisation drive that includes upgrading its nuclear weapons to not only deter foes but also be able to counter-attack if deterrence fails.

Experts say China's assessment of what constitutes a credible nuclear deterrent may also be changing -- and that substantial upgrades to its forces will embolden its military, particularly in regards to self-ruled Taiwan, which Beijing claims as its territory, and the disputed South China Sea, the majority of which China also claims.