North Korea has conducted three underground, nuclear test explosions since 2006. The latest, in February 2013, prompted international condemnation and a tightening of sanctions by the U.N. Security Council. The sanctions are intended to hinder the North's development of nuclear weapons and ballistic missiles.
The North's weapons capabilities are a subject of conjecture, but experts doubt it has yet mastered how to miniaturize a nuclear device that fit on a long-range missile capable of hitting the United States, although it is possible it may have for shorter-range missiles.
Associated Press writer Hyung-jin Kim in Seoul, South Korea, contributed to this report.
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