South Korea''s national security adviser left Tuesday for the United States for talks focusing on North Korea''s nuclear weapons programs, a diplomatic source said, amid a flurry of diplomacy to reopen the stalled multilateral forum on ending the North''s nuclear drive. Chun Yung-woo, senior presidential secretary for foreign affairs and national security, will meet with US National Security Adviser Tom Donilon and other senior US officials, South Korea''s News Agency (Yonhap) reported.