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.

WSJ: North Korea Plans Nuclear Detonation

South Korea has said North Korea cannot unilaterally dissolve the armistice and has called on Pyongyang to tone down its rhetoric.

BBC: South Korean prime minister visits border island

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.

CNN: 5 things we still don't know about North Korea's nukes

China Foreign Ministry spokesman Hong Lei said at a daily news briefing that China has "expressed our regret" over Pyongyang's declaration.

WSJ: North Korea to Restart Yongbyon Nuclear Plant

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