South Korea's nuclear envoy Sunday expressed pessimism about upcoming six-nation talks on North Korea's nuclear program as negotiators arrived in Beijing to prepare for the meetings. Kim Sook, who leads the South Korean delegation, was to meet his US and Japanese counterparts later Sunday to lay the groundwork for the latest round of talks scheduled to start Monday. North Korea - which conducted a nuclear test in 2006 - agreed last year to disable its nuclear reactor in exchange for aid. But it recently denied having agreed to allow inspectors to take samples from its nuclear complex to verify past nuclear activities. The discussions in the coming week are expected to focus on how to verify Pyongyang's accounting of the program, but negotiators have said they expect the process to be difficult. "I am not optimistic at all," Kim said, according to South Korea's Yonhap news agency. On Saturday, Kim met top US nuclear negotiator Christopher Hill in Seoul after Hill held two days of preliminary talks with his North Korean counterpart in Singapore.