The North does, however, appear to be making some progress.
Japan's Defense Ministry, in an assessment of the December launch presented to the prime minister on Friday, said the North's best designs probably give its missiles a range of more than 10,000 kilometers (6,200 miles), according to Japan's Kyodo news service. That would be enough to reach the West Coast of the United States. A South Korean defense official said Friday that Seoul agrees with that assessment.
The Japanese report warned that Pyongyang's missile technology has "entered a new stage" that is of serious concern to the international community. Japan is particularly wary of North Korea's capabilities because all of its islands are well within striking distance. Japan also hosts about 50,000 U.S. troops, whose bases would be a tempting target if Pyongyang were to try to make good on its threats.
"There has been a tendency to underestimate what North Korea can do in the space and missile field, and possibly with technology in general," U.S. nuclear expert Jeffrey Lewis wrote recently on his Arms Control Wonk blog. He noted that debris recovered from the wreckage of the December rocket's first stage indicates that most of it was made in North Korea.
North Korea claims the right to build nuclear weapons as a defense against the United States, which stations more than 28,000 troops in South Korea.
It is believed to have enough weapons-grade plutonium for about four to eight bombs, according to nuclear scientist Siegfried Hecker, who visited North Korea's nuclear complex in 2010. And in 2009, Pyongyang also declared that it would begin enriching uranium, giving it a second way to make atomic weapons.
U.S. Defense Secretary Leon Panetta said Thursday that he has seen no outward sign that North Korea will follow through soon on its plan to conduct a test, but added that doesn't mean preparations aren't under way.
A U.S. research institute said Friday that recent satellite photos of the Punggye-ri site where nuclear tests were conducted in 2006 and 2009 reveal that over the past month roads have been kept clear of snow and that North Koreans may have been sealing the tunnel into a mountainside where a nuclear device would be detonated.
The analysis was provided to The Associated Press by 38 North, the website of U.S.-Korea Institute at Johns Hopkins School of Advanced International Studies. The latest image was taken Wednesday.
38 North concludes that the Punggye-ri site "appears to continue to be at a state of readiness that would allow the North to move forward with a test in a few weeks or less once the leadership in Pyongyang gives the order."
U.S. officials confirmed Friday that the U.S. has seen some trucks moving around the site. One official said the U.S. is not ruling out that the test could happen in the near future.
But the officials cautioned that, as in previous tests, because it would be done underground, the U.S. may not know much before it actually happens. Officials spoke on condition of anonymity because they were not authorized to discuss intelligence matters publicly.
Associated Press writer Matthew Pennington contributed.
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