South Korea's chief nuclear negotiator left for the United States on Sunday to discuss ways to reopen the stalled six-party talks on ending North Korea's nuclear weapons program, the foreign ministry here said. During his five-day trip to Washington, Cho Tae-yong, Seoul's lead envoy to the six-party talks, plans to meet with his U.S. counterpart, Glyn Davies, and other National Security Council officials to exchange views on the issue, the ministry said.