Sung Kim is leading the team of US experts in Pyongyang
US experts have made a "good start" to the process of dismantling North Korea's main nuclear facility, the leader of the US team has said.
Sung Kim praised North Korean officials at the Yongbyon reactor, which produced weapons-grade plutonium, as being "very co-operative".
Pyongyang agreed to end its nuclear programme in return for diplomatic concessions and economic aid.
US officials say they hope to disable the reactor by the end of the year.
"I think we are off to a good start," said Mr Kim. "I hope to achieve all the disablement, at least this phase of disablement, by 31 December."
The plant has already been closed, and the measures taken by the US team are reversible. But they will put the reactor out of action for at least a year.
Working in two or three-week rotations over about two months, the US experts are expected to disable the plant by removing the 8,000 fuel rods that power the reactor.
The agreement on disabling came after four years of negotiations involving China, the US, Japan, Russia and the two Koreas.
North Korea has also agreed to give a complete declaration of its nuclear programmes by the end of the year.
The Yongbyon facility is believed to have produced the material for a test device detonated a year ago, proving the regime's nuclear capability.
US Assistant Secretary of State Christopher Hill has said Washington will work closely with North Korea on removing it from the US list of state sponsors of terrorism.
The facility at Yongbyon, about 100km (60 miles) north of Pyongyang, was set up in the 1960s at the height of the Cold War.