Nine nuclear-armed nations including the US, Russia, France, China, India and Pakistan, continued to modernise their nuclear arsenals and several of them deployed new nuclear-capable weapon systems in 2023, according to the Stockholm International Peace Research Institute(SIPRI)in its report released on Monday.

The report said China's nuclear arsenal increased from 410 warheads in January 2023 to 500 in January 2024, and it is expected to keep growing.  The report put India's 'stored' nuclear warheads at 172 in January this year while the number for Pakistan was 170.

"While Pakistan remains the main focus of India's nuclear deterrent, India appears to be placing growing emphasis on longer-range weapons, including those capable of reaching targets throughout China," the report said.

India slightly expanded its nuclear arsenal in 2023, it said, adding that both India and Pakistan continued to develop new types of nuclear delivery systems in 2023.

The report said some 2,100 of the deployed warheads were kept in a state of high operational alert on ballistic missiles, and nearly all of them belonged to Russia or the US. However, for the first time China is believed to have some warheads on high operational alert, it said.

The SIPRI said nine nuclear-armed states -- the US, Russia, the UK, France, China, India, Pakistan, North Korea and Israel -- continued to modernise their nuclear arsenals and several deployed new nuclear-armed or nuclear-capable weapon systems in 2023.

Of the total global inventory of an estimated 12,121 warheads in January 2024, about 9,585 were in military stockpiles for potential use, it said.

An estimated 3,904 of those warheads were deployed with missiles and aircraft -- 60 more than in January 2023 -- and the rest were in central storage, it said.

"Around 2,100 of the deployed warheads were kept in a state of high operational alert on ballistic missiles. Nearly all of these warheads belonged to Russia or the US, but for the first time China is believed to have some warheads on high operational alert," the report said.

According to the Swedish think-tank, India, Pakistan and North Korea are all pursuing the capability to deploy multiple warheads on ballistic missiles, something Russia, France, the UK, the US and more recently China already have.

This would enable a rapid potential increase in deployed warheads, as well as the possibility for nuclear-armed countries to threaten the destruction of significantly more targets, it said.

The SIPRI said Russia and the US together possess almost 90 per cent of all nuclear weapons.

The sizes of their respective military stockpiles 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, it 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, it added.

The report said 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 US by the turn of the decade.

At the same time, the report said China's stockpile of nuclear warheads is still expected to remain much smaller than the stockpiles of either of Russia and the US.

"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," Kristensen said.