A team of US technicians is due to begin disabling North Korea's nuclear complex on Monday with Pyongyang's permission, a top US envoy has said.
Sung Kim is leading the team of experts in Pyongyang
The technicians have left Pyongyang and are due to arrive at the main nuclear site, Yongbyon, on Sunday, according to Christopher Hill.
Pyongyang agreed to disabling in return for aid and security guarantees.
It shut down its reactor at Yongbyon in July and is to give details of its programme by the end of the year.
Speaking in Tokyo, Mr Hill said the US experts would start disabling the reactor, North Korea's only functioning one, and two other facilities on Monday.
"It's a very big day because it's the first time it [North Korea] is actually going to start dismantling its nuclear programme," he added.
Two months' work
The team, headed by Sung Kim, director of the US State Department's Office of Korean Affairs, reached Pyongyang on Thursday afternoon via Beijing.
"We would like to start as soon as possible," Mr Kim said.
The agreement on disabling came after many rounds of six-party negotiations in Beijing involving China, the US, Japan, Russia and the two Koreas.
Earlier, Mr Hill said he was "very satisfied" with progress made in preparatory talks with his North Korean counterpart, Kim Kye-gwan.
Kim Kye-gwan confirmed there were "no major differences in opinion".
Working in two or three-week rotations over about two months, the US experts are expected to slowly disable the plant, which produces weapons-grade plutonium.
"It will be a combined effort, with North Korean help and our experts supervising and co-ordinating," said Mr Kim.