South Korean government analysis has put North Korea's nuclear spending at USD 1.1 billion to USD 3.2 billion overall, although experts say it is impossible to make an accurate calculation given the secrecy surrounding the programme, and estimates vary widely.

However, the weapons that North Korea has tested thus far are comparatively small and based mostly on less sophisticated fission, or atomic bomb, technology.

The isolated North's claim that its fourth and most recent test, conducted last week, was of a more advanced and powerful hydrogen bomb has been widely doubted, although experts said it is possible Pyongyang took the intermediate step of boosting an atomic bomb with hydrogen isotopes.

A former South Korean official involved in nuclear diplomacy with North Korea told Reuters previously that it was likely the North's nuclear programme was cutting corners on safety, further driving down costs.

North Korea was at the bottom of a 2011 list on nuclear arms spending by Global Zero, a group campaigning to rid the world of nuclear weapons.

The full cost of Pyongyang's programme that year was estimated by the group at USD 700 million, making it the lowest spender among nuclear states, beneath Pakistan's estimated USD 2.2 billion, although the analysis was made before the North's two most recent nuclear tests.

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