The chief UN nuclear inspector, in Pyongyang for talks on how North Korea will close its main atomic reactor, was unable to meet with the country's top nuclear negotiator Wednesday because of scheduling problems, the agency's spokeswoman said. Mohamed ElBaradei, head of the Vienna-based International Atomic Energy Agency, had been slated to meet with North Korean Vice Foreign Minister Kim Kye Gwan, IAEA spokeswoman Melissa Fleming said in a telephone interview. The trip was a significant first step towards renewed relations between the IAEA and the North, which kicked out the agency's inspectors in late 2002. North Korean officials said Kim "was busy preparing for the six-party talks," Fleming said from Pyongyang. She would not release any other details but said ElBaradei met instead with another vice foreign minister of the same rank, Kim Hyong Jun. It was not immediately clear if the change in schedule was a setback to denuclearization efforts, but Fleming said everything else was "going as scheduled" so far. U.S. Assistant Secretary of State Christopher Hill, the top American nuclear negotiator, told reporters after arriving in Beijing that he didn't know why the meeting was canceled but called it a "good sign" that North Korea received ElBaradei. Hill said talks in Beijing with ElBaradei were likely on Thursday, one day after his return from Pyongyang. The chief UN inspector also met with the chairman of North Korea's atomic energy authority on Tuesday and was scheduled to see Kim Yong Dae, vice president of the standing committee of the Presidium of the Supreme People's Assembly, North Korea's legislature, on Wednesday, Fleming said. So far, talks have been centered around "general relations between the IAEA and (North Korea), including their membership status and, of course, the role for the IAEA under the initial actions," she said. Fleming was referring to conditions under a milestone Feb. 13 deal in which North Korea has 60 days to shut down and seal its Yongbyon nuclear reactor and a reprocessing facility that is believed to have provided enough plutonium for North Korea's estimated four to 13 nuclear bombs. The IAEA is supposed to monitor and verify the shutdown. The agreement was struck at the last round of international nuclear disarmament talks involving the two Koreas, the United States, Japan, Russia and host China. Delegates from those countries were set this week to meet with their counterparts to discuss economic and energy cooperation, peace and security in Northeast Asia and the denuclearization of the Korean Peninsula as part of working group sessions established under the landmark pact. Those meetings were to take place through the weekend. The next full session of six-nation nuclear disarmament talks was scheduled to begin Monday in Beijing. Also Wednesday, South Korea expressed optimism that the February agreement will be implemented, including shutting down Yongbyon, the North's only operating nuclear reactor, by April 14 in return for energy aid and political concessions. "I believe that the agreements will be implemented," Foreign Minister Song Min-soon told reporters at a regular briefing. Song said ElBaradei would brief related countries' officials after returning to Beijing on Wednesday night. The North Korean nuclear crisis began in 2002, when Washington alleged that Pyongyang had a uranium enrichment program in addition to its acknowledged plutonium program. North Korea then withdrew from the Nuclear Nonproliferation Treaty and expelled ElBaradei's inspectors. The North later restarted its main reactor at Yongbyon, and is believed to have produced enough plutonium in recent years for as many as a dozen nuclear bombs - including the weapon it detonated in an underground test blast on Oct. 9. Song said there are currently no signs of change in the operation of the Yongbyon nuclear reactor. The North is to eventually receive 1 million tons of heavy fuel oil for abandoning all its nuclear programs. US officials have stressed this must include an alleged uranium enrichment program, which the North has never publicly admitted having.