Five nations are officially recognised as possessing nuclear weapons by the 1968 nuclear Non-Proliferation Treaty (NPT).
Pakistan, like India and Israel, is not party to the NPT
These are the US, the first to acquire nuclear capability in 1945, Russia (1949), the UK (1952), France (1960) and China (1964).
As information about nuclear arsenals is secret, there are only estimates about their nuclear weapons.
The Arms Control Association (ACA), a US weapons research
organisation, estimates the number of strategic warheads held by these states to be about 6,000 for the US, 5,000 for Russia, 300 for China, 350 for France and under 200 for the UK.
The NPT, which has 187 signatories, was created to prevent other countries from acquiring nuclear capability, to promote co-operation in the peaceful use of nuclear energy and to work towards nuclear disarmament.
The International Atomic Energy Agency (IAEA) was set up to verify compliance to the treaty through inspections, making sure that nuclear material or activities were not being used for military purposes.
Three states - India, Israel and Pakistan - never joined the treaty and are known to possess nuclear weapons.
Claiming its nuclear programme was for peaceful purposes, India first tested a nuclear explosive device in 1974.
India and Pakistan both demonstrated their nuclear weapons muscle with a round of tit-for-tat tests in May 1998.
While Israel has not publicly conducted a nuclear test and does not admit or deny having nuclear weapons, it is widely believed to possess nuclear arms.
The ACA estimates India to have between 45 and 95 nuclear warheads, Pakistan 30 to 50, and Israel 75 to 200.
Iran is generally perceived to be secretly pursuing a nuclear arms programme although it maintains its nuclear work is for peaceful purposes.
The EU is engaged in negotiations with Tehran to urge it to suspend its nuclear activities while the US has stated that it will not allow Iran to acquire nuclear weapons.
North Korea, which this week announced it was pulling out of multilateral talks on its nuclear activities, is believed by the CIA to have one or two nuclear weapons.
The ACA says Pyongyang also has sufficient spent nuclear fuel that could be reprocessed into fissile material for as many as six nuclear bombs.
The CIA has raised concerns that Syria, which is an NPT member, could be trying to covertly acquire nuclear bombs given the availability of foreign expertise.
One of the most recent successes in non-proliferation was in 2003 when Libya renounced its then secret efforts to acquire nuclear capability.
ESTIMATED NUCLEAR WARHEADS, STRATEGIC AND TACTICAL
*The US is also said to have some 3,000 warheads in reserve, while Russia has about 11,000 in non-operational stockpiles
Israel declines to confirm it has nuclear weapons
North Korea claims it has nuclear arms but no details are available
Iran is accused by the US of ambitions to build nuclear arms