The UN chief Ban Ki Moon said the North Korea "crisis had gone too far" after Pyongyang announced plans to restart its nuclear reactor at Yongbyon.
Speaking at a news conference Ban said:
"Things must begin to calm down, there is no need for the DPRK [North Korea] to be on a collision course with the international community. Nuclear threats are not a game."
The Yongbyon complex was closed down in July 2007 as part of the disarmament-for-aid deal, which later stalled.