"The sizes of their respective military stockpiles (i.e. useable warheads) seem to have remained relatively stable in 2023, although Russia is estimated to have deployed around 36 more warheads with operational forces than in January 2023," read the statement issued by the think tank.

The report said transparency regarding nuclear forces has declined in both countries in the wake of Russia’s full-scale invasion of Ukraine in February 2022, and debates around nuclear-sharing arrangements have increased in saliency.

Nearly all of these warheads belonged to Russia or the USA, but for the first time China is believed to have some warheads on high operational alert.

"While the global total of nuclear warheads continues to fall as cold war-era weapons are gradually dismantled, regrettably we continue to see year-on-year increases in the number of operational nuclear warheads," said SIPRI Director Dan Smith. "This trend seems likely to continue and probably accelerate in the coming years and is extremely concerning."

The report said nine nuclear-armed states—the United States, Russia, the United Kingdom, France, China, India, Pakistan, the Democratic People’s Republic of Korea (North Korea) and Israel—continued to modernize their nuclear arsenals and several deployed new nuclear-armed or nuclear-capable weapon systems in 2023.

SIPRI’s estimate of the size of China’s nuclear arsenal increased from 410 warheads in January 2023 to 500 in January 2024, and it is expected to keep growing.

For the first time, China may also now be deploying a small number of warheads on missiles during peacetime.

Depending on how it decides to structure its forces, China could potentially have at least as many intercontinental ballistic missiles (ICBMs) as either Russia or the USA by the turn of the decade, although its stockpile of nuclear warheads is still expected to remain much smaller than the stockpiles of either of those two countries.

"China is expanding its nuclear arsenal faster than any other country," said Hans M. Kristensen, Associate Senior Fellow with SIPRI’s Weapons of Mass Destruction Programme and Director of the Nuclear Information Project at the Federation of American Scientists (FAS).

"But in nearly all of the nuclear-armed states there are either plans or a significant push to increase nuclear forces," Hans M. Kristensen said.

North Korea continues to prioritize its military nuclear programme as a central element of its national security strategy.

SIPRI estimates that the country has now assembled around 50 warheads and possesses enough fissile material to reach a total of up to 90 warheads, both significant increases over the estimates for January 2023. While North Korea conducted no nuclear test explosions in 2023, it appears to have carried out its first test of a short-range ballistic missile from a rudimentary silo.

It also completed the development of at least two types of land-attack cruise missile (LACM) designed to deliver nuclear weapons.

"Like several other nuclear-armed states, North Korea is putting new emphasis on developing its arsenal of tactical nuclear weapons," said Matt Korda, Associate Researcher with SIPRI’s Weapons of Mass Destruction Programme and Senior Research Fellow for the Nuclear Information Project at the Federation of American Scientists. "Accordingly, there is a growing concern that North Korea might intend to use these weapons very early in a conflict."