Christopher Hill told reporters he hoped to see Yongbyon disabled and to have a full declaration of its nuclear programmes by the end of the year, including how much fissile material it has, as well as "a very clear situation on the uranium enrichment".
Hill, who briefed the US president on the agreement on Tuesday, said the US anticipated having people on the ground in North Korea "heavily involved" in the final phase of disarmament.
If approved by all six parties and carried out by North Korea, the agreement would mark a step toward the US goal of getting North Korea to abandon all its nuclear weapons and programmes.
The US assertion that North Korea has a uranium enrichment programme sparked the latest nuclear crisis with Pyongyang in 2002.
At the time, Washington said North Korean officials had acknowledged the programme but Pyongyang has since denied it.
Hill, however, suggested that the two sides were making some headway.
"We have had, I think, some intensive and productive discussions on ... the issue of resolving our concerns about uranium enrichment," he said.