At the start of the year, China had raised its number of nuclear warheads to 250 from 240 in 2012 as part of a process to modernise its defense, the Stockholm International Peace Research Institute said.
However it also said China is "highly non-transparent" when it comes to its nuclear arsenal.
SIPRI researcher Pillip Schell said China may continue "the very slight process of increasing" its nuclear arms in the long-term but that there is no sign the country will change its current policy of maintaining a credible deterrent with the smallest possible nuclear arsenal.
Bitter rivals Pakistan and India also increased their arsenals by around 10 warheads each, the think-tank said.
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Meanwhile, the US and Russia continued their disarmament in accordance with the new START treaty that they signed in 2010.
The US reduced its number of warheads to 7,700 from 8,000, and Russia cut its arsenal to 8,500 from 10,000.
However, at the same time the US, Russia, France, China and Britain have all either deployed new nuclear weapon delivery systems or announced programs to do so, SIPRI said.
"The long-term modernization programs under way in these states suggest that nuclear weapons are still a marker of international status and power."
Here is SIPRI's list of the number of nuclear warheads in the world at the start of 2013 compared with the start of 2012.