It's a pretty exclusive group, the collection of countries officially recognized as nuclear weapons states. There are only five: the United States, Russia, the United Kingdom, France and China.

They were deemed to be Nuclear Weapons States under the Nuclear Non-proliferation Treaty, which came into being at the height of the Cold War on July 1, 1968. The United Nations treaty was designed to keep a lid on the spread of nuclear weapons - and to prevent countries other than the five that had tested nuclear weapons to that point from acquiring the technology needed to produce nuclear warheads.

At the same time, article four of the treaty states that any country that signed on had the "inalienable right" to research, produce and use nuclear energy for peaceful purposes. Nuclear weapons states were prevented from transferring nuclear weapons - or nuclear weapons technology - to other countries, just as non-nuclear states that had signed the treaty would not be allowed to try to acquire weapons or weapons technology from nuclear states.

The treaty came into force on March 5, 1970 when enough countries had signed on. Twenty-five years later, signatories to the treaty agreed to extend it indefinitely. The vast majority of independent states - 188 - have signed on. However, there are three notable exceptions: India, Pakistan and Israel.

India and Pakistan have tested nuclear devices. Israel will neither confirm nor deny that it possesses nuclear weapons.

North Korea signed the treaty on Dec. 12, 1985. But on Jan. 10, 2003, it served notice that it was pulling out of the treaty. On Oct. 9, 2006, the Communist dictatorship announced that it had tested a nuclear weapon in an underground site near the city of Kilju, near the country's east coast.

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Nuclear Weapons States

The world's first atomic explosion was conducted on July 16, 1945, on what is now known as the White Sands Missile Range in New Mexico. (AP Photo)

United States
The U.S. was the first country to develop and test a nuclear warhead. The Manhattan Project was set up during the Second World War to make sure the U.S. developed an atomic bomb before Nazi Germany did. The U.S. remains the only country to have used an atomic weapon. It dropped two of them — in August 1945 — on Japan in the final days of the war.

Russia
In 1949, Russia - then the Soviet Union - successfully tested a nuclear bomb of its own. It's believed that at least some of the technology the Soviets relied on was acquired through spying on the U.S. The Soviets were determined to acquire nuclear capability to counter the perceived U.S. threat.

U.K.
On Oct. 3, 1952, Britain successfully tested a nuclear weapon of its own. Operation Hurricane was launched to counter the perceived Soviet threat in Europe. The U.K. decided that it did not want to have to rely on the United States for its security. Some of the plutonium used in the device was supplied by Canada.

France
France tested its first nuclear warhead in 1960, also to counter the perceived Soviet threat in Europe. France did not want to have to rely on the U.K. or the U.S. for its security.

China
China was a surprise entry to the nuclear club in 1964 when it tested its first warhead. It had long looked to the Soviet Union for help in developing its nuclear technology. But by the late 1950s, relations between the two Communist powers were deteriorating, so China decided to develop its own arsenal to counter the perceived threat from the Soviet Union and the West.

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Countries that haven't signed but have tested weapons

India
On May 18, 1974 - less than two years after then prime minister Indira Gandhi authorized scientists to build a bomb - India set off the "Smiling Buddha." India called it a "peaceful nuclear explosive." Pakistan called it a threat. India relied on technology supplied by a nuclear reactor that Canada helped to build.

Pakistan
Not long after India's nuclear test, Pakistan turned its attention to developing a device that would negate the military advantage India had acquired. Pakistan turned to China for help. It's believed Pakistan's nuclear weapons were ready sometime in the 1980s - but the first confirmed test did not take place until 1998.

North Korea
North Korea signed the NPT on Dec. 12, 1985. On Jan. 10, 2003, it announced its intention to pull out - a month after announcing it had violated its 1994 agreement to freeze its nuclear weapons program. On April 24, 2003, North Korea announced that it possessed a nuclear bomb - but the claim was not verified. On Oct. 9, 2006, the country announced that it had tested a nuclear weapon in an underground site near the city of Kilju, near the country's east coast.

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Non-signatory countries believed to possess warheads

Israel
Israel has never confirmed that it has nuclear weapons. It's never denied it, either.

In 1958 - with French help - it began work on a nuclear reactor near the town of Dimona in the Negev desert. Israel said it needed to develop nuclear power for a desalination plant to "green" the area.

The reactor was online by 1964. When the U.S. asked Israel to allow international inspections of the reactor, it agreed, but under two conditions: that inspections be carried out by Americans and that Israel be given notice of inspections.

The U.S. stopped inspecting the plant in 1969, saying the exercise was useless.

In 1986, Mordechai Vanunu - a former technician at the plant - leaked evidence of Israel's nuclear program to the media. He was tried in secret and jailed for 18 years.

Countries that say they're ready - but haven't conducted tests

Iran
The newest wannabe member of the club announced on April 11, 2006 that it had enriched uranium for the first time, a critical step in the path towards making a bomb. But President Mahmoud Ahmadinejad said his country does not aim to develop nuclear weapons.

A day later, the country's deputy nuclear chief said Iran intends to enrich uranium on a scale hundreds of times larger than its current level. The UN Security Council has demanded that Iran stop all enrichment activity.

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Countries that had the technology but gave it up

Ukraine
Ukraine inherited nuclear warheads when the Soviet Union broke up in 1991. It signed on to the Nuclear Non-proliferation Treaty and agreed to rid itself of warheads.

Belarus
Belarus inherited nuclear warheads when the Soviet Union broke up in 1991. It signed on to the Nuclear Non-proliferation Treaty and agreed to rid itself of warheads.

Kazakhstan
It inherited nuclear warheads when the Soviet Union broke up in 1991. Kazakhstan signed on to the Nuclear Non-proliferation Treaty and agreed to rid itself of warheads.

South Africa
South Africa is the only nation to have successfully developed nuclear weapons and then got rid of them. Former president F.W. de Klerk announced that his country had produced six nuclear bombs, but destroyed its arsenal before it ratified the NPT on July 10, 1991.

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Countries that pursued nuclear weapons programs at one time

Argentina
Argentina actively pursued a nuclear weapons programme in the 1980s, but abandoned it after the election of a democratic government. By 1990, Argentina signed a treaty for a nuclear weapons-free zone in Latin America.

Brazil
Brazil actively engaged in a nuclear weapons program in the 1980s, but shut it down before developing weapons. It also signed the 1990 Latin America weapons-free zone treaty.

Algeria
In 1991, Algeria was found to be building a reactor capable of producing weapons-grade material. It placed the reactor under International Atomic Energy Agency safeguards and signed on to the NPT.

Iraq
By 1981, Iraq was believed to be on the verge of nuclear weapons capability - until Israeli fighters jets destroyed the Osiraq reactor. The country's defeat in the 1991 Gulf War put an end to Iraq's nuclear ambitions. No trace of a resumed nuclear-weapons program was found in the aftermath of the U.S.-led invasion that toppled Saddam Hussein in 2003.

Libya
Libya was one of the first countries to sign the NPT. But in 2003, the North African country announced that it had tried to develop nuclear weapons but had abandoned the program and rid itself of all weapons of mass destruction. It was part of the country's bid to end UN sanctions and its isolation from the world community.

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