SEOUL: South Korea hosts some 50 world leaders at a two-day nuclear security summit starting on Monday to discuss ways to safeguard nuclear materials and facilities from terrorist groups.
It is the second such summit - the first was hosted by U.S. President Barack Obama in Washington in 2010.
Here is a look at official and unofficial nuclear powers and their weapon stocks:
OFFICIAL NUCLEAR POWERS
* UNITED STATES: The United States used nuclear weapons on Hiroshima and Nagasaki in August 1945 at the end of World War Two, making it the only country ever to use nuclear weapons during a conflict.
-- Under a new treaty, a successor to the 1991 Strategic Arms Reduction Treaty (START), the United States and Russia are to limit the number of operationally deployed nuclear warheads to 1,550 - 30 percent lower than the ceiling of the 2002 Treaty of Moscow established for each side. The new treaty entered into force in February 2011.
-- The 2002 treaty (the Strategic Offensive Reductions Treaty, or SORT) stated that each country must reduce their deployed strategic nuclear forces to 1,700-2,200 warheads by 2012.
-- According to the START counting rules, as of January 2009 the United States had an estimated 5,113 nuclear warheads.
-- The United States had historically deployed nuclear weapons in six European NATO countries as part of its commitment to extended deterrence. In 2010 there were approximately 200 U.S. tactical nuclear weapons deployed in Belgium, Germany, Italy, the Netherlands, and Turkey. However, it removed its nuclear weapons from Germany in July 2007, and from Britain in June 2008.
-- Obama's "Prague Speech" in April 2009 committed the United States to the long-term goal of zero nuclear weapons.
* RUSSIA: The Soviet nuclear weapon programme began during World War Two and culminated in an atomic bomb test in 1949.
-- By October 2010, Russia's strategic nuclear arsenal had been reduced to approximately 2,660 warheads. Further reductions are required under the new START treaty, ratified by the Russian Duma in January 2011.
-- However, Moscow's current total stockpile of strategic and tactical nuclear warheads remains uncertain as there is no accurate count of tactical nuclear weapons. Russia inherited a massive nuclear weapons production complex and large stocks of weapons grade fissile material. It is estimated by NTI that Russia currently has approximately 770 metric tons of weapons grade-equivalent highly enriched uranium (HEU) and approximately 128 metric tons of military-use plutonium.
* FRANCE: France detonated its first nuclear bomb in February 1960, and carried out its final nuclear tests in January 1996 - tests at sites in the Sahara and on Pacific atolls.
-- France has been a party to the Treaty on the Non-Proliferation of Nuclear Weapons (NPT) since 1992. In 2008, President Nicolas Sarkozy announced the country would leave its submarine missile arsenal in place while cutting its stock of air-launched weapons by a third to around 290 warheads.
-- As of September 2008, France had already pared its arsenal to about 300 nuclear warheads.
* BRITAIN: The current nuclear stockpile consists of fewer than 180 strategic warheads that can be deployed on four Vanguard-class nuclear-powered ballistic missile submarines (SSBN).
-- The 2010 Defence Review further reduced the maximum number of deployed missiles to 40 per submarine, part of a plan to limit operational nuclear weapons to 120 within coming years.
-- Because the first two submarines of the existing Vanguard-class fleet are due to be retired in 2024, in early 2007 British lawmakers gave their initial support to a plan for designing a new class of replacement nuclear-armed submarines. However due to defence budget cuts, the final decision on the new delivery platform has been delayed until 2016.
* CHINA: China's nuclear weapons programme began in 1955 and it has since conducted 45 nuclear tests, including tests of thermonuclear weapons and a neutron bomb.
-- China closely guards information about its nuclear arsenal. However, the U.S. Department of Defense has said that China has approximately 130-195 deployed nuclear-capable ballistic missiles.
-- Although not official, China is reported to have placed a moratorium on fissile material production. The International Panel on Fissile Materials estimated that China produced around 20 metric tons of HEU, and still holds around 16 metric tons.
-- It acceded to the NPT in 1992 as a nuclear weapon state.
OTHER "DECLARED" NUCLEAR NATIONS:
* NORTH KOREA: North Korea promised to abandon its nuclear programme in 2005 but later backed away from the agreement, testing nuclear devices in October 2006 and May 2009.
-- North Korea is believed to have enough fissile material to make up to a dozen nuclear bombs, and in November 2010 unveiled a uranium enrichment facility in addition to its plutonium programme, opening a second route to making a weapon.
-- Six-party talks between North Korea, South Korea, Japan, China, Russia, and the United States began in 2003 with the goal of denuclearizing the Korean peninsula. The talks were suspended in April 2009 after the North launched a ballistic missile.
-- North Korea this month announced a plan to launch a satellite with a long-range rocket, casting doubts on possible new talks following the death of Kim Jong-il and the ascent of his young and untested heir, Kim Jong-un.
* INDIA: India embarked on a nuclear explosives programme in 1968, culminating in a May 1974 test of a "peaceful nuclear explosion." Following five nuclear tests in May 1998, India formally declared itself a nuclear weapon state.
-- As of 2010, India had produced between 60 and 80 nuclear devices, with not more than 50 of these operational. Estimates in 2010 put India's HEU stockpile at between 0.2 and 0.5 tons, and its weapons-grade plutonium at between 0.3 and 0.7 tons.
* PAKISTAN: In the mid-1970s, Pakistan took the uranium enrichment route to acquiring nuclear weapons capability under the direction of A.Q. Khan. By the mid-1980s, Pakistan had a clandestine uranium enrichment facility. Pakistan conducted nuclear tests in May 1998 shortly after India, and declared itself a nuclear weapon state.
-- According to 2011 estimates by the International Panel on Fissile Materials, Pakistan has accumulated a stockpile of around 2.75 tons of HEU. U.S. intelligence estimates in 2011 put the number of deployed weapons at around 90 to 110.
-- Neither India nor Pakistan are signatories to the NPT.
OTHER UNDECLARED NATIONS:
* ISRAEL: Israel is understood to possess a sizeable nuclear arsenal, but neither confirms nor denies this under a "strategic ambiguity" policy to deter Arab and Iranian adversaries.
-- Israel, like India, Pakistan and North Korea, is outside the NPT but unlike the others, it has never publicly tested nuclear devices.
-- Israel has two nuclear reactors, the secretive Dimona facility in the Negev desert, where it is assumed to have produced its nuclear arsenal, and a research reactor, open to IAEA inspection, at Nahal Soreq near Tel Aviv.
-- Based on estimates of the plutonium production capacity at Dimona, Israel is believed to have manufactured enough fissile material for between 100 and 300 nuclear warheads.
* IRAN: Iran has been a non-nuclear-weapon state party to the NPT since 1970. It has a uranium enrichment programme which it says is to produce energy. Western powers suspect Iran is trying to develop the means to make atomic bombs because of its past failure to declare nuclear facilities to the U.N. nuclear watchdog and continued restrictions on U.N. inspections.
-- Iran is under U.N. Security Council sanctions for refusing to suspend uranium enrichment. A 2011 IAEA report did reveal a trove of intelligence pointing to research activities in Iran of use in developing the technology needed to assemble nuclear weapons. AGENCIES