North Korea is expected to begin disabling its nuclear facilities Monday, marking the biggest step the communist country has ever taken to scale back its atomic program.

The North shut down its sole functioning nuclear reactor at Yongbyon in July, and promised to disable it by year's end in exchange for energy aid and political concessions.

Disabling the reactor at Yongbyon, north of Pyongyang, would mark a further breakthrough in efforts to convince the North to scale back its nuclear program. The country conducted its first-ever nuclear test in October 2006.

"By Monday morning, they will begin their work," U.S. Assistant Secretary of State Christopher Hill said in Tokyo on Saturday, referring to the U.S. team that arrived in Pyongyang last week. "It's a very big day because it's the first time it's actually going to start disabling its nuclear program."

South Korean nuclear envoy Chun Yung-woo said Monday that the U.S. team will determine which specific disablement measures it will take first after considering technical issues and safety concerns. Last week, South Korean Foreign Minister Song Min-soon told reporters the team will carry out about 10 measures to disable the Yongbyon facilities.

Chun said he had not confirmed that the U.S. team started the disabling work as scheduled.

Hill said the U.S. intends to achieve the denuclearization of the Korean peninsula while President Bush is still in power, and that North Korea — one of the world's most isolated countries — appeared to be opening up.

"I'd like to see us get through this in the current U.S. administration," Hill told a news conference in Tokyo. "We started this process, and I'd like to see us finish it."

To disable the program, the facilities must be stripped sufficiently that it would take at least a year for North Korea to start them up again, Hill said.

Hill added the U.S. hoped to disable North's uranium enrichment program by Dec. 31, not just its plutonium-production facilities at Yongbyon.

"By the end of the year ... we hope to have arrived at an important milestone, where there is a complete disablement of the Yongbyon facilities, a full list of additional facilities for disablement, and that uranium enrichment is also resolved to mutual satisfaction," Hill said.

The envoy said American lawyers were working with North Korea to prepare to remove it from a U.S. list of states sponsoring terrorism, but that Pyongyang ultimately needed to meet requirements stipulated under U.S. law.

Taking Pyongyang off the terror list, long a key demand of the North, was one of a series of economic and political concessions offered for the country to disable its nuclear reactor that produces plutonium for bombs.