It rebuffed efforts by Secretary of Defense Robert Gates to develop a next-generation nuclear warhead.
It is not thought to have a missile capable of carrying a nuclear warhead.
The fear was like that of a nuclear warhead falling into the wrong hands.
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Foreign Secretary William Hague has said Iran has been testing missiles capable of carrying a nuclear warhead.
In 2007 America concluded, controversially, that Iran had been developing a nuclear warhead for a missile, but had stopped in 2003.
Its ratification depends on there being confidence that the US nuclear warhead stockpile can be safely and effectively maintained without test detonations.
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Mr. Hagel didn't specify when U.S. military and intelligence analysts believe Pyongyang will have an ICBM capable of carrying a nuclear warhead.
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That, Mr Netanyahu said, represented the point when Iran would have sufficient 20%-enriched uranium to produce enough of the weapons-grade variety needed for a nuclear warhead.
The United States and other Western nations have demanded that Iran stopped its uranium-enrichment program, which they perceive as a precursor to production of nuclear warhead-grade material.
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.
The recent report by the director of the International Atomic Energy Agency (IAEA), Yukiya Amano, raised serious concern over Iran's actions to develop a nuclear warhead for its missiles.
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%.
Friday's successful advanced missile defense test, which shot down a mock nuclear warhead in space, shows that the US is getting closer to defending the country against ballistic missile attack.
But some of these -- some of these programs, for instance, the replacement nuclear warhead or shutting Yucca Mountain -- you can't really say that Yucca Mountain is an ineffectual program.
The US and other regional allies fear North Korea is working to develop a nuclear warhead small enough to arm a missile, though it is not believed to have achieved this yet.
Mr. Hagel did not say when the Pentagon believes North Korea will have an ICBM that can carry a nuclear warhead, but said the Pentagon would stay ahead of Pyongyang's military developments.
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Regional neighbours and the US fear Pyongyang is working to build a nuclear warhead small enough to put on a missile, but believe it does not yet have the capabilities to do so.
The U.S. has said North Korea does not possess a nuclear warhead or the required technology to miniaturize one to fit on its rockets, but it could put a "dirty bomb" or conventional weapon on one.
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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.
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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While U.S. officials said there is "nothing to indicate" that North Korea can or will place a nuclear warhead atop a missile right now, most experts say it is very difficult to know the warhead and payload of a missile test in advance.
Defense officials now believe that North Korean missiles can reach Hawaii as well as Alaska, but not the continental U.S. At the same time, U.S. officials don't believe North Korea has developed a miniaturized nuclear warhead that could be mounted on any missile.
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And then there is the nuclear complex where warhead components are manufactured, refurbished and dismantled.
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