State Department spokesman Sean McCormack told reporters he could not confirm news reports that North Korea had broken seals placed by the International Atomic Energy on equipment at its Yongbyon nuclear plant.

But McCormack reiterated that the North Koreans were putting equipment that had been in storage back to former positions.

"They're actually taking some of the steps that would allow them to restart; and actually restarting, actually being able to use Yongbyon as it was previously configured and for its previous purposes," he said.

"I don't think at this point that you can say they are at that last step. But they are, it would appear, starting to take some of those initial actions that would allow them to get to the third phase," he said.

He said there were significant costs of restarting the plant, "not only diplomatically and politically, but also just retooling, getting the facility back up and operating again."

Added to that were questions of whether or not the North Koreans have the technical expertise and resources for restarting the plant.

North Korea last week announced that it has stopped work on disabling Yongbyon, and would consider rebuilding the plants, because Washington has failed to drop it from a terrorism blacklist.

US nuclear negotiator Christopher Hill was in Beijing at the weekend consulting some of his counterparts in the six-party talks about getting denuclearization back on track.

Hill said Saturday that the United States will take North Korea off its terror list "immediately" if it can agree a way to verify its nuclear facilities.

The Yongbyon reactor is at the heart of Pyongyang's decades-old nuclear weapons drive and produced the plutonium for its October 2006 atomic test.

Last year North Korea sealed a landmark deal with the United States, China, South Korea, Japan and Russia to abandon all its nuclear weapons in exchange for badly needed energy and economic aid and security and diplomatic benefits.