North Korea, which said it tested an atomic weapon in October 2006, has agreed to start disabling its plutonium-producing plants under a six-nation accord which also requires it to declare all nuclear programmes.
Hill, the chief US envoy to the six-nation talks, said the country appeared to be making greater efforts to open up.
"In the past North Korea often spoke of their isolation as a great benefit ... I think they've understood it now as something that is actually harming them"
Christopher Hill, US assistant secretary of state
"Whereas in the past North Korea often spoke of their isolation as a great benefit for their country, I think they've understood it now as something that is actually harming them," he said.
The North has promised to disable the plants and declare a list of all programmes including a suspected highly enriched uranium project by year-end.
"The idea of disablement is to create a situation where it is very difficult to bring those facilities back on line and certainly a very expensive and difficult prospect of ever bringing them back on line," Hill said.
Under the February accord North Korea will receive energy aid worth hundreds of millions of dollars in return for disablement.
If it goes on next year to dismantle the plants and give up its plutonium, North Korea can expect normalised relations with Washington. The country also hopes to be taken off a US list of state sponsors of terrorism.
"They [North Korea] have to address the terrorism concerns that put them on the list in the first place," Hill said.
"When there are countries on the terrorism list it needs to be understood that they are on the list for a reason. They are on the list because they've been supporting terrorism in some respect," he added.
|The US says it is concerned that N Korea could transfer nuclear technology to Syria|
Washington is also concerned about the risk of North Korea sharing its nuclear technology, Hill said, following earlier unconfirmed reports that the communist state was helping Syria to develop a nuclear programme.
"Proliferation has been a primary concern of ours all along," he said.
Hill said that although "he has received assurances that they will not transfer" nuclear technology, he will remain to watch "closely areas of concern" including Syria.
The US envoy was in Tokyo for talks with his counterpart in the six-party talks, after visiting China and South Korea earlier in the week.