"It is simply a matter of scheduling and logistics," Tom Casey, a US State Department spokesman, told reporters.

It is not clear when the next round of talks, usually held in Beijing, will take place.

On Wednesday the top US negotiator to the six-party process, Christopher Hill, left North Korea saying efforts to disable its plutonium-producing reactor are "going well and on schedule".

However he added that the work needed to continue if deadlines are to be met.

Hill, an assistant US secretary of state, spent two days in North Korean and visited the Yongbyon nuclear complex, becoming the highest-level American official to go there.

"I'm satisfied with the results," Hill told reporters before leaving Pyongyang en route to Beijing.

"But we have to keep working because we have more to do to meet our deadlines."

Key topic

Last week Hill said he expected North Korea to submit the list of its nuclear program to Beijing, the host of the six-party talks, before the end of this week.

The talks bring together envoys from the US, Russia, China, Japan and the two Koreas.

Hill said the North Korean declaration would have to include all material, including a plutonium stockpile and any warheads.

On Tuesday Cho Hee-Yong, spokesman for South Korea's foreign ministry, said he understood that Pyongyang had not yet handed over the list of its nuclear programs, which was to have been a key topic of the six-party talks originally expected to start Thursday.

"Considering the time needed, it appears it would be difficult to hold a new round of the six-party head of delegation talks this week," Cho said.