Pyongyang has said it is willing to allow South Korean businessmen to visit the jointly-run Kaesong zone.
U.S. experts have described the North's satellite as tumbling in space and said it does not appear to be functioning, though Pyongyang has said it is working.
Pyongyang has previously said it is willing to restart the six-party talks, which also include China, Japan, Russia and South Korea.
Although the reactor has been idled, Pyongyang has never publicly said it stopped enriching uranium at the site after revealing the enrichment facility there in 2010.
U.S. Defense Secretary Leon Panetta said Pyongyang has been "needlessly provocative, " and that the U.S. is fully prepared for any action by North Korea.
South Korea has said North Korea cannot unilaterally dissolve the armistice and has called on Pyongyang to tone down its rhetoric.
He said Pyongyang was also claiming it has made a smaller nuclear bomb that could be delivered by missile.
The Pentagon announcement, analysts said, will send a clear signal to Pyongyang that it has Washington's attention, and that the West intends to respond.
WSJ: U.S. to Boost Missile Defenses in Response to North Korea
Mike Chinoy, a former CNN correspondent who has visited North Korea 15 times, said the current confusion about Pyongyang's capabilities recall a similar episode in 1998, when U.S. spy satellites discovered an underground complex at Kumchangri, not far from North Korea's main nuclear facility at Yongbyon.
China Foreign Ministry spokesman Hong Lei said at a daily news briefing that China has "expressed our regret" over Pyongyang's declaration.
But she said that the American intelligence community didn't have a unified position on just how far Pyongyang has progressed on this track.
WSJ: Clinton Praises China Stimulus, Has Hope for North Korea Relationship
North Korea has warned that the situation has grown so tense it cannot guarantee the safety of foreigners in the country and said embassies in Pyongyang should think about their evacuation plans.
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