U.S. State Department official Sung Kim says North Korea will complete nuclear disablement by year's end.

Arriving in Beijing after a three-day visit to the North to monitor the disablement process, Kim told reporters Thursday the trip was very useful and productive. The State Department's point man on Korea inspected three key nuclear facilities in Yongbyon as part of an international team of officials and nuclear experts.

Kim also said that Pyongyang has yet to declare its nuclear programs, adding that the latest trip focused on disablement and not on the declaration issue.

Meanwhile, South Korean official Lim Sung-nam, who heads the Foreign Ministry's task force on the North Korean nuclear issue, also confirmed that the disablement process is going smoothly.