China was the only country in the world to increase the size of its nuclear arsenal last year, according to new research.
Beijing reportedly added 10 warheads to its nuclear programme in 2014, bringing its total to an estimated 260 weapons, according to the Stockholm International Peace Research Institute (SIPRI).
Due to China's notoriously secretive stance on its nuclear programme, it is not known how many of its warheads are operational. An operational warhead is one placed on a missile or supported by operational forces, meaning they could be quickly deployed in the event of nuclear warfare.
China reportedly spent $7.6bn (€6.8bn) on its nuclear weapons programme in 2011. Though this represents a tiny fraction of the US expenditure in the same year - $61.3bn (€54.7bn) - Washington is gradually reducing the number of warheads it has, just as Beijing appears to be moving in the opposite direction.
Elsewhere, the total number of warheads in the world fell by almost 7,000 between 2010 and 2015, largely due to reductions in the US and Russian arsenals. Despite these cuts, the two countries still have by far the largest arsenals. Moscow possesses an estimated 7,500 warheads, while the US has 7,260.
The total number of deployed warheads is 4,300 out of an overall total of 15,850.
The SIPRI report warned that, despite an overall trend towards disarmament, all nine states studied are believed to be modernising and upgrading their nuclear arsenals.
In the UK, which has approximately 215 warheads, David Cameron's Conservative government is committed to replacing the country's Trident nuclear programme. The British Ministry of Defence estimates the cost of building four new nuclear submarines at up to £23.4bn (€32.3bn). However, protest group the Campaign for Nuclear Disarmament puts the total cost at around £100bn (€138.2bn).
France, which has 300 warheads, is currently modernising both air and sea-based forces at an annual cost of €180bn until 2019.
The other countries in the research, India, Pakistan, Israel and North Korea, are all believed to be deploying new weapons systems now or in the near future. For India and Pakistan, this could involve expanding production capabilities and building more efficient missile delivery systems.
Israel refuses to either confirm or deny whether it possesses nuclear weapons and the size of any such arsenal.
The study said North Korea's nuclear progress was difficult to assess due to a lack of open sources. However, recent missile tests suggest Pyongyang is developing a capacity for submarine-launched ballistic missiles (SLBM), which would greatly increase its nuclear threat by allowing warheads to be deployed outside the Korean peninsula.
Only five states, the US, Russia, the UK, France and China, are internationally authorised to possess nuclear weapons by the 1968 Non-Proliferation Treaty (NPT). Representatives of these five states, plus Germany and the European Union, are currently trying to hammer out the technical details of a deal limiting Iran's nuclear capabilities before an end of June deadline.Try Newsweek: Subscription offers