The top US nuclear negotiator for North Korea visited its reactor, becoming the highest-level American official to go there, an official said Tuesday, amid progress in efforts to end the communist nation's decades-old pursuit of atomic weapons. Assistant Secretary of State Christopher Hill inspected work at the Yongbyon complex, north of Pyongyang, Monday afternoon, said Max Kwak, a spokesman for the US Embassy in Seoul. Kwak said Hill would meet his North Korean counterpart, Vice Foreign Minister Kim Kye Gwan, on Tuesday. He gave no more details, including Hill's reaction to the ongoing work to disable the plutonium-producing reactor being conducted under a six-nation agreement. On Tuesday morning, Hill toured Pyongyang and rode a subway with other members of the US delegation, including Sung Kim, the State Department's top Korea expert, according to footage from broadcaster APTN. Hill also met with North Korean Foreign Minister Pak Ui Chun, the North's official Korean Central News Agency reported in a brief one-sentence dispatch. Hill's visit to Yongbyon made him the highest-level US official to visit the complex, according to the US Embassy in Seoul. The compound is at the heart of North Korea's pursuit of nuclear weapons that culminated in its first-ever nuclear test explosion in October last year. North Korea began disabling the reactor, which was shut down in July, and two other facilities last month under the watch of US experts. The disablement, which will make the reactor difficult to restart, is the biggest step the communist nation has taken to scale back its nuclear programs. Under the six-nation agreement - signed by the two Koreas, the US, China, Japan and Russia - North Korea is to receive energy aid and diplomatic concessions in return. North Korea had promised to complete the disabling by the end of the year, but South Korean nuclear envoy Chun Yung-woo said last week it would take longer to remove about 8,000 spent fuel rods from the reactor. North Korea is also required to declare all its nuclear programs by year's end, and the issue was expected to top Hill's talks with his North Korean counterpart. Hill said last week that Pyongyang was finalizing the declaration of its nuclear programs and that he would talk about the document with the North Koreans. The declaration will serve as a map of all North Korea's nuclear programs that Washington hopes can be dismantled by the end of 2008. Following Hill's trip to the North, the six-nation talks were expected to resume in Beijing. The US envoy visited the North once before in June, becoming the first high-level US official to visit the secretive country in more than four years.