The North has evaded a decade of UN sanctions to develop the uranium enrichment process, enabling it to run an effectively self-sufficient nuclear programme that is capable of producing around six nuclear bombs a year, they said.

The true nuclear capability of the isolated and secretive state is impossible to verify. But after Pyongyang conducted its fifth and most powerful nuclear test last week and, according to South Korea, was preparing for another, it appears to have no shortage of material to test with.

North Korea has an abundance of uranium reserves and has been working covertly for well over a decade on a project to enrich the material to weapons-grade level, the experts say.

That project, believed to have been expanded significantly, is likely the source of up to 150 kilograms (330 pounds) of highly enriched uranium a year, said Siegfried Hecker, a leading expert on the North's nuclear programme.

That quantity is enough for roughly six nuclear bombs, Hecker, who toured the North's main Yongbyon nuclear facility in 2010, wrote in a report on the 38 North website of Johns Hopkins University in Washington published on Monday.

Added to an estimated 32- to 54 kilogram plutonium stockpile, the North will have sufficient fissile material for about 20 bombs by the end of 2016, Hecker said. North Korea said its latest test proved it was capable of mounting a nuclear warhead on a medium-range ballistic missile, but its claims to be able to miniaturise a nuclear device have never been independently verified.

Assessments of the North's plutonium stockpile are generally consistent and believed to be accurate because experts and governments can estimate plutonium production levels from telltale signs of reactor operation in satellite imagery. South Korean Defence Minister Han Min-koo this year estimated the North's plutonium stockpile at about 40 kilogrammes.

But Hecker, a former director of the US Los Alamos National Laboratory, where nuclear weapons have been designed, has called North Korea's uranium enrichment programme "their new nuclear wildcard," because Western experts do not know how advanced it is.

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