The nuclear Non-Proliferation Treaty (NPT), ratified in 1970, was designed to keep atomic weapons from spreading beyond the five major powers who then had atomic weapons -- the United States, Russia, China, Britain and France. It forbade these countries to transfer nuclear weapons technology, and other signatories from attempting to acquire or produce nuclear bombs.

For more than a decade, proliferation was held in check, but in 1998 India and Pakistan held nuclear tests two weeks apart.

Israel is also believed now to have the bomb, although its official policy is one of strategic ambiguity in which it neither confirms nor denies its nuclear military potential.

Analysts are divided as to whether North Korea's efforts to develop nuclear weapons have yielded concrete results.

Iran is the most recent possible nuclear aspirant to generate headlines, particularly since it came under investigation in February 2003 by the Vienna-based International Atomic Energy Agency (IAEA), which monitors compliance with the NPT.

The United States asserts Iran is secretly developing nuclear weapons and has demanded that Tehran give up all its uranium enrichment activities.

Washington would like the IAEA to refer the case of Iran's suspected nuclear weapons program to the UN Security Council for possible sanctions.

In July, Israeli intelligence chiefs told Prime Minister Ariel Sharon's security cabinet in a joint assessment that Iran will have a nuclear weapons capacity by 2007, according to public radio reports.

Other analysts project similar time frames.

Benn Tennenbaum, an expert at the Federation of American Scientists, thinks that "it will take several years" for Iran to develop a bomb if they only use the gas centrifuge program that is known to be in place.

But he said that "if they have a separate (hidden) program, they would probably do it very quickly," adding that such a covert program would be outside IAEA inspections.

Analysts note that even without a covert program, nations can develop the know-how to make atomic weapons while strictly adhering to the NPT, which does not ban enriching uranium, for example.

IAEA director general Mohamed ElBaradei has called for an international effort to fix the loopholes in the NPT.

North Korea is an example of a country which left the NPT once it had achieved a technological level sufficient to make nuclear weapons.

In early 2003 Pyongyang withdrew from the NPT and expelled IAEA inspectors, leading the agency to refer the case of North Korea to the Security Council, though the UN has refrained from imposing sanctions.

Meanwhile, South Korea has recently admitted that it carried out uranium enrichment and plutonium experiments in the 1980s banned by the treaty.

The IAEA did refer Libya to the UN Security Council earlier this year, but the action was a pro-forma move insofar as Tripoli had renounced at the end of 2003 all its programs of weapons of mass destruction and cooperated in an IAEA investigation after that.

There have been other such success stories.

After the fall of former communist dictator Nicolae Ceausescu, Romania's new leaders alerted the United Nations to possibly suspect activities, while Argentina and Brazil both renounced any nuclear ambitions before signing the

South Africa was described by the IAEA as a "model of cooperation" for its supervised nuclear disarmament at the end of apartheid in 1993-94.

Kazakhstan, Belarus and Ukraine all agreed to ban any stockpiling of nuclear weapons on their national territory following the breakup of the Soviet Union in 1991.

Meanwhile, experts note that several highly-industrialized nations -- Germany and the Netherlands, for example -- have sufficiently developed technological infrastructures to quickly development nuclear weapons if they so decided.

"I would be concerned about Japan," Tennenbaum said. "They have a very large nuclear reactor complex and have enormous stockpiles of spent fuel, more than enough to build many bombs," he said.

"But they certainly don't have the desire to build it right now."

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