North Korea is cooperating with US experts to disable its nuclear weapons-making facilities under an accord with Washington and regional powers and should be able to complete the process by the end of the year, a US diplomat said Tuesday. "I think we are off to a good start," Sung Kim, the State Department's top expert on Korea, said after arriving in South Korea after a visit to North Korea's Yongbyon nuclear site. Kim said North Korean officials were "very cooperative" and that disablement work had begun at three major facilities at the main Yongbyon nuclear complex, located 60 miles north of Pyongyang. That includes a 5-megawatt reactor that can generate plutonium for bombs, and nuclear fuel fabrication and reprocessing plants. "I hope to achieve all the disablement, at least this phase of disablement, by Dec. 31," he said. The US and other countries have declined to publicly state how the North's nuclear weapons facilities will be disabled. But the main U.S. envoy to arms talks with the North, Assistant Secretary of State Christopher Hill, has said the experts would take steps that would mean it would take at least a year for the reactor to be restarted. The North shut down its sole operational reactor at Yongbyon in July and promised to disable it by year's end in exchange for energy aid and political concessions from other countries involved in talks on its nuclear program: the U.S., China, Japan, South Korea and Russia. Washington hopes future talks will yield an agreement for the North to dismantle the facility entirely, and also wants the nuclear bombs Pyongyang is believed to have built to be confiscated. The country conducted its first-ever nuclear test detonation in October 2006 - the culmination of decades of efforts to build the world's deadliest weapons - and experts estimate it has enough weapons-grade plutonium to make about a dozen bombs. Kim traveled to Seoul to take part in meeting of U.S. and South Korean defense ministers Wednesday, the U.S. Embassy in Seoul said. Defense Secretary Robert Gates was arriving in Seoul from China for previously planned discussions on the US-South Korea alliance. Some 29,000 US troops remain deployed in the South as a legacy of the Korean War, which ended in 1953 cease-fire that has never been replaced by a peace treaty.