More recently, they tested a maneuvering ballistic missile warhead capable of hitting U.S. aircraft carriers in the Western Pacific.
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Trying to shoot down an incoming missile or warhead with a physical projectile, by contrast, is much more difficult.
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.
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.
The latest, smaller version of the missile does have a warhead but is meant to destroy its target primarily by simply hitting it.
Pentagon officials promised to press forward with plans for more tests of the system after an interceptor missile hit a dummy warhead over the Pacific Ocean on Saturday.
While true, North Korea's intimate ties with Iran and Syria show that North Korea's nuclear program, with its warhead, missile and technological components, is not a distant threat, limited in scope to faraway East Asia.
In 2007 America concluded, controversially, that Iran had been developing a nuclear warhead for a missile, but had stopped in 2003.
Pyongyang also is seen as being ahead of Iran in developing the technologies needed to place an atomic warhead on a missile.
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.
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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We don't even know whom to target, obsessing over President Ahmadinejad when in fact he does not control the things that concern us most: the Revolutionary Guard, Hezbollah, support for the insurgents in Iraq and Afghanistan to kill our people, the ballistic missile program or the nuclear warhead program.
And they have also worked on designing at least of applying this missile to carry a nuclear warhead. 10.23.58 Narrator According to reported Israeli intelligence, the newer versions of the Shihab would reach not just Israel but Western Europe. 10.24.05 Professor Isaac Ben-Israel A nuclear warhead is something which is effective against cities, against populated areas.
It is not thought to have a missile capable of carrying a nuclear warhead.
India has successfully launched a long-range intercontinental ballistic missile able to carry a nuclear warhead, officials say.
On Thursday, a U.S. lawmaker revealed at a congressional hearing in Washington an excerpt from a recent U.S. intelligence report compiled by the Pentagon's Defense Intelligence Agency, which concluded with "moderate confidence" that North Korea had mastered the ability to affix a miniaturized nuclear warhead to a ballistic missile.
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.
The Pentagon estimates that Iran could test-flight an intercontinental missile, capable of carrying a nuclear warhead, as early as 2015.
The second is to instill fear in the American public by one day demonstrating its capability to marry a nuclear warhead with an intercontinental ballistic missile with the range to hit the U.S. West Coast.
This and its own snooping suggest military links to the nuclear programme, secretive procurement efforts, high-explosive testing useful in triggers for nuclear bombs, and design work on a missile cone able to carry a nuclear warhead.
The AP reported that the nuclear experts at the IAEA were in agreement that Iran already possesses the capability to build a nuclear bomb and is "on its way" to developing a missile system capable of delivering a nuclear warhead.
If a smaller device was indeed tested, analysts said this could take Pyongyang closer to building a warhead small enough to arm a missile.
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.
North Korea claimed that a "miniaturised" device had been tested, increasing fears that Pyongyang had moved closer to building a warhead small enough to arm a missile.
Pyongyang claims its nuclear test involved a smaller and more powerful device - prompting concerns it could be moving closer to creating a warhead small enough to arm a missile.
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%.
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.
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.
BBC: North Korea in nuclear warning after UN rocket resolution
But by the afternoon, the IAEA had rained all over the Krewe of Obama-Gates parade and warned the world that Iran was likely to overcome the challenges of putting a nuclear warhead on top of a ballistic missile delivery vehicle.
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