China said Thursday it has been working hard to restart long-stalled six-nation negotiations on North Korea's nuclear program, with one of its senior diplomats set to visit South Korea this week immediately after leaving the North. The back-to-back trip by China's Vice Foreign Minister Liu Zhenmin to the two Koreas is highly unusual and believed to be part of U.S.-involved diplomacy to revive the six-party talks. Wrapping up a four-day visit to Pyongyang on Thursday, Liu will arrive in Seoul later in the day.