Campaigners have warned that the expiry of the New START treaty on Thursday, which ended decades of restrictions on how many warheads the two powers deploy, could encourage China to expand its own arsenal.

The United States has said any new nuclear agreement would have to include China, but international efforts to encourage Beijing to join fresh talks have so far failed.

"China has always maintained that the advancement of arms control and disarmament must adhere to the principles of maintaining global strategic stability," Beijing foreign ministry spokesman Lin Jian told a news conference.

"China's nuclear capabilities are of a totally different scale as those of the United States and Russia (and) will not participate in nuclear disarmament negotiations at this stage," he said.

The New START treaty ended at the turn of the calendar on February 5, after US President Donald Trump did not follow up on Russian counterpart Vladimir Putin's proposal to extend warhead limits in the agreement for one year.

Russia and the United States together control more than 80 percent of the world's nuclear warheads but arms agreements have been withering away.

China's nuclear arsenal is growing quickly, with an estimated 550 strategic nuclear launchers, which is still well below the 800 each at which Russia and the United States were capped under New START.

New START, first signed in 2010, limited each side's nuclear arsenal to 1,550 deployed strategic warheads -- a reduction of nearly 30 percent from the previous limit set in 2002.

It also allowed each side to conduct on-site inspections of the other's nuclear arsenal, although these were suspended during the Covid-19 pandemic and have not resumed since.