The top nuclear envoys of South Korea and China discussed North Korea's recent nuclear threat and agreed to bolster "strategic cooperation" to prevent Pyongyang from carrying out its fourth nuclear test, a Seoul diplomat said Saturday. Hwang Joon-kook, Seoul's chief envoy to the six-party talks that also involve the United States, North Korea, Russia and Japan, held talks with his Chinese counterpart, Wu Dawei, in Beijing on Friday. The meeting came on the heels of three-way talks with the U.S. and Japan in Washington earlier this week.