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.
India has successfully launched a long-range intercontinental ballistic missile able to carry a nuclear warhead, officials say.
Yet there are other indications including possible design work on a nuclear warhead that its intentions are far from peaceful.
Moreover, according to South Korean military officials, the North is on the verge of completing a "Super-EMP" nuclear warhead.
Experts, however, doubt the regime has managed to develop a nuclear warhead small enough to mount on its long-range missiles.
The Pentagon estimates that Iran could test-flight an intercontinental missile, capable of carrying a nuclear warhead, as early as 2015.
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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Libya not only had names of contacts to find answers to after-sales scientific questions, but also the blueprint for a nuclear warhead.
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.
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.
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.
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.
Combined with predictions that Iran could manufacture a nuclear warhead within two years, a new consensus of opinion could be built up to justify attacking Iran.
One U.S. intelligence assessment suggested North Korea had the capacity to put a nuclear warhead on a missile, even if any such weapon would have low reliability.
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.
In July, it brought into service a new missile capable, in theory, of carrying a nuclear warhead as far as Israel, or indeed reaching American bases in the Middle East.
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.
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