This comes as state-controlled media in North Korea said leader Kim Jong Un has made an "important" decision regarding the nation's nuclear policy following a meeting with a large group of military leaders.
The North said Jan. 24 it would detonate a third weapon in an underground test facility in response to a United Nations Security Council resolution condemning its Dec. 12 long-range rocket test.
China, North Korea's closest ally in the region, is said to be working behind the scenes to get North Korea to scrap the planned nuclear test, South Korea's Yonhap News Agency said Sunday. Though China has yet to acknowledge it publicly, a high-level envoy may travel to Pyongyang to urge Kim not to set off the bomb.
South Korean President Lee Myung-bak urged military and civilian leaders to be vigilant against aggression from the North and made an unscheduled visit to his security headquarters to examine the nation's ongoing efforts to monitor its reclusive neighbor.