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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This includes tests of high-explosive triggers useful for nuclear weapons, and the redesign of a missile nose-cone to take what looks like a nuclear warhead.
India has successfully launched a long-range intercontinental ballistic missile able to carry a nuclear warhead, officials say.
The Pentagon estimates that Iran could test-flight an intercontinental missile, capable of carrying a nuclear warhead, as early as 2015.
North Korea already has the ability to deliver a nuclear warhead: it has an extensive missile programme and once test-fired a missile over part of Japan.
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%.
He says North Korea is years away from developing a nuclear warhead that would be small, light and heat-resistant enough to launch on a missile.
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.
North Korea provoked Western condemnation earlier this month with an underground nuclear test that was preceded by the December launch of a long-range missile capable of transporting a 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.
Worse yet, evidence continues to accumulate that both North Korea and Iran are interested in a nuclear weapons application that poses a particular and extraordinary danger to the United States: a ballistic missile-delivered warhead that would cause an electro-magnetic pulse (EMP) capable of inflicting what a blue-ribbon commission recently called "catastrophic" damage to the electric grid and electronic systems across the country.
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