By Johan Ahlander

STOCKHOLM (Reuters) -The world's nuclear-armed states are beefing up their atomic arsenals and walking out of arms control pacts, creating a new era of threat that has brought an end to decades of reductions in stockpiles since the Cold War, a think tank said on Monday.

Of the total global inventory of an estimated 12,241 warheads in January 2025, about 9,614 were in military stockpiles for potential use, the Stockholm International Peace Research Institute said in its yearbook, an annual inventory of the world's most dangerous weapons.

Around 2,100 of the deployed warheads were kept in a state of high operational alert on ballistic missiles, nearly all belonging to either the U.S. or Russia.

SIPRI said global tensions had seen the nine nuclear states - the United States, Russia, the United Kingdom, France, China, India, Pakistan, North Korea and Israel - plan to increase their stockpiles.

"The era of reductions in the number of nuclear weapons in the world, which had lasted since the end of the Cold War, is coming to an end," SIPRI said. "Instead, we see a clear trend of growing nuclear arsenals, sharpened nuclear rhetoric and the abandonment of arms control agreements."

SIPRI said Russia and the U.S., which together possess around 90% of all nuclear weapons, had kept the sizes of their respective useable warheads relatively stable in 2024. But both were implementing extensive modernization programmes that could increase the size of their arsenals in the future.

The fastest-growing arsenal is China's, with Beijing adding about 100 new warheads per year since 2023. China could potentially have at least as many intercontinental ballistic missiles as either Russia or the U.S. by the turn of the decade.

According to the estimates, Russia and the U.S. held around 5,459 and 5,177 nuclear warheads respectively, while China had around 600.

(Reporting by Johan AhlanderEditing by Peter Graff)