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“I don’t credit that at all,” Mr. Gates said in Bolivia, where he is attending a regional defense conference, adding that North Korea had a nuclear arms program for some time and probably had a number of nuclear devices.

It wasn’t immediately clear why the North chose to reveal the previously hidden facility. It could be a ploy to win concessions in nuclear talks or an attempt to bolster leader Kim Jong-il’s apparent heir, son Kim Jong-un.

Analysts also speculated that North Korea is seeking to gain leverage in any aid-for-disarmament negotiations in stalled six-way talks with regional powers China, Japan, South Korea, Russia and the United States.