North Korea will have enough material for about 20 nuclear bombs by the end of this year with enhanced uranium-enrichment facilities and an existing stockpile of plutonium, according to new assessments by weapons experts.

The revelations came as North Korea accused the United States of pushing the Korean peninsula to "the point of explosion" after it dispatched two huge bombers in a show of force against North Korea.

The American supersonic B-1B Lancers flew over South Korea on Tuesday as the US pledged its "unshakeable commitment" to defend its allies in the region, following North Korea's fifth and largest-ever nuclear test conducted last week.

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The North has evaded a decade of United Nations sanctions to develop its uranium enrichment process, enabling it to run an effectively self-sufficient nuclear programme that is capable of producing six nuclear bombs a year, arms analysts say.

The true nuclear capability of the isolated and secretive state is impossible to verify.

According to South Korea, the North is preparing for another nuclear test - a sign it has no shortage of material to do so.

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

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That project, believed to have been expanded significantly, is most probably the source of up to 150kg 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 DC, published on Monday.

Added to an estimated 32-54kg plutonium stockpile, the North will have sufficient fissile material for about 20 bombs by the end of 2016, Hecker said.

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.

Nuclear wild card

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 wild card".

Jeffrey Lewis, of the California-based Middlebury Institute of International Studies, said North Korea had an unconstrained source of fissile material, both plutonium from the Yongbyon reactor and highly enriched uranium from at least one and probably two sites.

"The primary constraint on its programme is gone," Lewis said.

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Weapons-grade plutonium has to be extracted from spent fuel taken out of reactors and then reprocessed, and therefore would be limited in quantity.

A uranium-enrichment programme greatly boosts production of material for weapons.

Despite sanctions, by now North Korea is probably largely self-sufficient in operating its nuclear programme, although it may still struggle to produce some material and items, Lewis said.

"While we saw this work in Iran, over time countries can adjust to sanctions," he said.

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Source: Agencies