After the war, when Washington stationed nuclear warheads in South Korea and Japan, Pyongyang, received help from the Soviet Union and China, and launched its own drive to acquire the bomb.

Later, assistance was obtained in crucial technology areas from other countries, including Pakistan.

In the mid-1950s North Korea signed a research agreement with Moscow under which hundreds of its scientists were trained in nuclear physics by the Soviets. Pyongyang later signed a similar cooperation agreement with China.

Around 1960 North Korean leader Kim Il-Sung ordered the construction of an atomic energy research complex in Yongbyon, 90 kilometres (55 miles) north of Pyongyang.

Two years later Soviet scientists helped the North Koreans assemble a two-megawatt IRT-200 research reactor shipped by Moscow to Pyongyang. In return, North Korea exported the spent fuel back to the Soviet Union. The reactor was operation in 1965.

In 1974, North Korean leader Kim visited China, and reportedly won China's promise to train more North Korean nuclear scientists.

North Korea was ready to step up its nuclear drive and five years later began work on a second reactor at Yongbyon. The five-megawatt research reactor was operational in 1987, and ready to produce some seven kilogrammes of plutonium a year, enough for one or two nuclear weapons.

In 1989, US satellite pictures showed the existence of a reprocessing plant at the Yongbyon complex.

At about the same time, the reactor was shut down and North Korea was suspected of unloading it and reprocessing spent nuclear fuel rods to separate around 12 kilogrammes of plutonium, enough for a couple of atomic bombs.

Washington accused North Korea of actively pursuing nuclear weapons. Pyongyang denies the charge but started to build two larger reactors, a 50-megawatt and a 200-megawatt plant.

Gathering tension about North Korea's nuclear ambitions reached crisis point when the United States reported several undeclared nuclear sites in North Korea and Pyongyang rejected outside inspections and threatened to withdraw from the nuclear Non-Proliferation Treaty (NPT).

The two-year-long standoff was headed off in 1994 when Pyongyang agreed to freeze its nuclear programme in return for the construction of safe nuclear reactors for the impoverished country.

The deal, however, has failed to dispel suspicions that the Stalinist country was secretly developing nuclear weapons.

Finally it unravelled in October 2002 when Washington said North Korea, while freezing its plutonium-based programme, had admitted secretly using a different route to nuclear weapons, helped by Pakistan.

North Korea began seeking nuclear weapons fuel through uranium enrichment while the ink was still wet on the 1994 accord, according to US sources.

The program was given a boost by Abdul Qadeer Khan, the father of Pakistan's nuclear bomb, who in 2004 admitted proliferating nuclear technology to North Korea and other countries.

According to US reports he made 13 trips to North Korea in the 1990s and helped supply uranium enrichment equipment and possibly even warhead designs.

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