North Korea is cooperating with U.S. experts to disable its nuclear weapons-making facilities, a U.S. diplomat said Tuesday after a trip to the communist nation to oversee the start of the work this week.

Sung Kim, the State Department's top expert on Korea, said North Korean officials were "very cooperative" and that work on disablement had begun at three major facilities at the main Yongbyon nuclear complex, 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 think we are off to a good start and will look forward to completing the task by the end of the year as planned," Kim said after arriving in Seoul following a visit to Yongbyon.

"Our North Korean colleagues have actually done considerable preparatory work on all three facilities. So we were able to start at least some of the disablement activities this week," he said.

The U.S. and other countries have declined to publicly state how North Korea's nuclear weapons facilities will be disabled, only saying that about 10 technical measures will be taken to disable them. Kim said he expects one of the steps will be completed this week.

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.

State Department spokesman Sean McCormack said the disablement marked "groundbreaking territory."

"The world has not been to this point vis-a-vis the North Korean nuclear program before," he told reporters. If disablement is completed by year end, it would be "a real testament to the mechanism of the six-party talks as a way to bring about change in North Korean behavior," he said.

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 U.S.-South Korea alliance.

Some 29,000 U.S. 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.