North Korea closed the reactor at Yongbyon, located north of Pyongyang, in July. American officials believe the Yongbyon facility supplied the weapons-grade plutonium for North Korea's first nuclear test, conducted last year.
The move is one of the biggest steps the North has taken toward denuclearization, a stipulation reached during October's six-party talks with the United States, China, Japan, South Korea and Russia. North Korea agreed to the steps in exchange for economic aid and increased political recognition.
"It's a very big day because it's the first time [North Korea is] actually going to start disabling its nuclear program," Christopher Hill, Assistant Secretary of State for East Asian and Pacific Affairs, told the Associated Press.
Hill, who is also the head of the six-party talks, said that further requirements for disablement will continue until the end of the year, the deadline set by the negotiations.
"By the end of the year, on the road to denuclearization, 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, according to the AP.
Hill also said the United States will begin taking steps to remove North Korea from Washington's list of countries that support terrorism, one of the concessions granted to the communist country during negotiations. But Hill added that striking North Korea from the terror list would depend on whether the state completes full denuclearization and achieves compliance with U.S. law, the AP reported.
Disabling the Yongbyon reactor is expected to take about two months and will be conducted by teams of U.S. experts in two to three-week rotations, the BBC reported.