He later issued a terse statement pointing to the need for defences to deal with emerging long-range missile threats.
Analysts say North Korea is years away from being able to accurately deliver a nuclear weapon atop a long-range missile.
U.S. officials recently said they obtained evidence that North Korea was gearing up for a launch of a long-range missile.
U.S. and South Korean officials have said that North Korea seems to be preparing to test-fire its long-range missile, the Taepodong-2.
Most recently, the importance of America's alliance with Japan has been underlined by North Korea's preparations to test a long-range missile.
Military and intelligence officials in Seoul and Washington have been watching for signs that North Korea might soon test another long-range missile.
And a U.S. decision to postpone a long-range missile test this month could provide a pretext for the North to declare a symbolic victory.
The start of presidential campaigning in South Korea comes amid reports that North Korea could carry out a long-range missile test in coming weeks.
So I think that -- again, that was in direct response to the testing of a long-range missile in late March, early April -- I forget the exact date.
Two U.S. State Department officials, who spoke on condition of anonymity, said the long-range missile was the Taepodong-2, North Korea's most advanced missile with a range of up to 15, 000 kilometers 9, 320 miles.
For instance, a given long-range missile may have fewer warheads aboard it than the number it can carry but, in the interest of arms control monitoring, a larger number is automatically assigned to each missile of that type.
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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Even more alarmingly, Israel has no medium-range or long-range conventional missile arsenals.
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The U.S. and other countries, most notably Korea's close neighbor Japan, are deeply worried by the North's long-range ballistic-missile program, which Pyongyang has put on hold but not abandoned.
On Tuesday, Yediot Aharonot reported that Israeli analysts who reviewed videotapes of North Korea's missile tests concluded that alongside the various short range Scuds it sent over the Sea of Japan, Pyongyang also launched a Taeopodong-2 multi-stage long range missile capable of reaching Alaska.
Also, and especially in light of the proliferation of long-range missiles, theater and national missile defenses are key.
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Most analysts don't expect North Korea to stage an attack on the U.S. or its allies and it isn't thought to have the missile technology to hit long-range targets.
Security Council last year condemned North Korea's launch of a long-range rocket, saying it violated a resolution banning ballistic missile testing.
Kim Kwan-jin, South Korea's defense minister, said the latest missile didn't appear to be a long-range type.
The re-instituted missile ground-based system plan will put 14 more long-range interceptors in Fort Greely, Alaska by 2017 to supplement the 30 already existing on the West Coast.
They've said all along the threat is really Iran and Iranian potential long-range missiles, and that's why they wanted to expand the American missile defense system to Europe.
The Bush-era proposal called for the United States to set up a radar site in the Czech Republic and 10 missile interceptors in Poland to counter the threat of Iran launching long-range missiles at America's allies in Europe.
The threats against the U.S. are also hard to take seriously because hitting long-range targets is still widely believed to be beyond the current limits of North Korea's missile technology.
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