The US and North Korea's neighbours fear Pyongyang's ultimate goal is to put a nuclear warhead on a long-range missile that could target the west coast of the US, but it is not believed to have mastered the technology yet.

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The New Start treaty limits each side to no more than 800 deployed nuclear warhead delivery systems including bombers, missile launchers and nuclear submarines - a cut of about 50%.

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America, meanwhile, has again been showing around documents that purport to be Iranian design work on a missile nose-cone of a sort that that could carry a nuclear warhead.

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But a nuclear warhead would have to be small enough to fit onto, say, a Nodong missile - and it would have to be capable of withstanding the buffeting and other forces exerted on it during flight.

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