UN Chief Ban Ki-moon has said the North Korea "crisis has gone too far" after Pyongyang announced plans to restart its main Yongbyon nuclear complex.
Speaking at a news conference in Andorra, where he is on a visit, Mr Ban called for urgent talks with the North.
The move by Pyongyang is the latest in a series of measures in the wake of its third nuclear test in February.
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North Korea has been angered by the resultant UN sanctions and joint US-South Korea annual military drills.
A South Korean foreign ministry spokesman said that if true, the North Korean move - which includes reactivating a reactor mothballed for six years - would be "highly regrettable".
Chinese foreign ministry spokesman Hong Lei called for restraint from all sides to resolve the "complex and sensitive" situation.
The reactor at Yongbyon - which was the source for plutonium for North Korea's nuclear weapons programme — was closed in July 2007 as part of a disarmament-for-aid deal. — BBC