The comments by US officials contrasted with the assessent by countries such as Japan, which said it was "regrettable" the negotiations in Beijing broke for three weeks without achieving more results.

"We have made important progress in this round of talks," White House spokesman Trent Duffy told reporters in describing the six-nation negotiations aimed at persuading Pyongyang to renounce nuclear arms.

US officials said the talks, which resumed after a hiatus of 13 months, foundered on North Korea's insistence on keeping its civilian nuclear capacity.

But Duffy said there was a consensus on the need for a denuclearization of the Korean Peninsula and the Chinese asked for a break to give the negotiators time to return their capitals and discuss the next steps.

"We anticipate a reconvening of the talks on August 29th," Duffy said. "So we believe there has been important progress and we're still optimistic that some agreement can be reached."

Deputy State Department spokesman Adam Ereli also said "good progress was made" during the Beijing talks, even if North Korea's desire to keep light water reactors was a "non-starter."

He said the marathon round was marked by "sustained and substantive engagement on issues." The fact the parties went through four drafts of a statement of principles showed they were "moving the ball forward," he said.

"(We) couldn't get it over the goal line, but came darn close, and have, I think, maybe a good basis to think we can get there in a couple more weeks."