S Korea's top nuclear envoy to visit US this week

South Korea's top nuclear envoy will visit the United States this week, an official said Monday, amid reports of progress in North Korea's light-water reactor project that experts say may help expand the North's nuclear weapons capacity.    Lim Sung-nam, Seoul's chief negotiator to the six-party talks aimed at ending North Korea's nuclear weapons program, will leave for Washington on Tuesday and meet with his U.S. counterpart Glyn Davies and other officials during a three-day visit, the foreign ministry official said.    "During the visit, Lim will exchange views on the current situation in North Korea and recent developments in its nuclear issues with U.S. officials, and discuss ways for the two countries to work together," the official said on the condition of anonymity.    Early this month, Lim visited Beijing and held talks with his Chinese counterpart Wu Dawei. Seoul and Beijing are keeping a "close watch" on progress in North Korea's light-water atomic reactor project, Seoul officials said.    South Korea is concerned that the North's reactor under construction at its main nuclear complex in Yongbyon might be a cover to stockpile enriched uranium, a fissile material used to make bombs, although Pyongyang claims it is for producing electricity.    Late last month, the International Atomic Energy Agency said North Korea had made "significant" progress in the light-water reactor project. Citing satellite imagery, the U.N. said the North has put a dome over the facility.    The six-party talks, which involve the two Koreas, the United States, China, Russia and Japan, have been dormant since late 2008.