The United States said Monday that a team of U.S. experts had arrived at North Korea's sole functioning nuclear reactor and begun the work of disabling the facilities.

State Department spokesman Tom Casey told reporters that the disabling of the North's nuclear reactor at Yongbyon "is a positive first step in this process, and we certainly hope to see it continue."

He had no details about what specific steps the team was conducting. "This is going to be a process that is going to take some time," he said.

The North shut down Yongbyon in July and promised to disable it by year's end in exchange for energy aid and political concessions from other members of talks on its nuclear program: the U.S., China, Japan, South Korea and Russia.

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