After 13 days of negotiation, deadlocked disarmament talks on North Korea's nuclear weapons program have entered a three-week recess, with Washington and Pyongyang blaming each other for the impasse and urging the other to make concessions. Delegates insist a joint agreement can still be reached, but significant challenges remain.

The central issue in the talks remains the pace and extent to which North Korea might abandon nuclear weapons and technology.

North Korea has endorsed the principle of a nuclear-weapons-free Korean Peninsula, but says it wants the right to produce nuclear energy for peacetime use.

Pyongyang complicated the matter as the talks were winding down last week, unexpectedly demanding to be allowed to operate a type of nuclear power plant known as a light water reactor.

The United States says that in order to obtain the economic and diplomatic concessions it wants, North Korea has to give up all nuclear programs, including peacetime nuclear energy production. It says light water reactors, which can produce weapons-grade nuclear fuel, are definitely "not on the table."

But despite the deadlock, delegates say an agreement is not necessarily out of reach.

China's chief delegate to the talks, Wu Dawei, expressed optimism that a breakthrough could still be produced.

"There is no doubt that on some major issues there are differences among relevant parties, but we believe we are capable of overcoming these differences," he said.

Some diplomats at the talks said they might agree to Pyongyang retaining nuclear energy capacity, if it rejoins the nuclear Non-Proliferation Treaty and allows international inspectors back into the country.

Pyongyang pulled out of the treaty and expelled the inspectors in 2003, after the United States claimed the North Koreans had cheated on earlier agreements and begun a secret uranium-enrichment program.

During the current recess, the delegations will review Beijing's proposal for a joint statement, which outlines a set of principles that would serve as the basis for future talks.

Christopher Hill, the top U.S. envoy to the talks, says he hopes China will use the recess to encourage North Korea to accept the proposal, the fourth that Beijing drafted during the 13 days of talks.

Despite the impasse, Mr. Hill says the delegates did make progress these last two weeks.

"We have really had a dialogue, an understanding that we have not had before," he said. "What we have tried to do is reduce the differences and to understand and see with great precision what those differences are."

The United States also appears to have made concessions on several major issues.

North Korea entered the talks demanding significant economic aid and U.S. security guarantees in exchange for nuclear disarmament.

Mr. Hill told reporters Sunday the proposal now on the table would virtually solve Pyongyang's energy needs, and be a major step towards normalized relations between North Korea and the United States, Japan and South Korea.

Bruce Jacobs, a Korean affairs expert at Australia's Monash University, says the change in tone that Mr. Hill referred to, coming after three inconclusive rounds of talks, is encouraging.

"Both sides are starting to get to the stage where at least they can talk without huge threats to one another. This, it would seem to me, would be a positive factor towards a future solution," he said.

The U.S. and North Korean sides have confirmed that they will maintain "contact" during the recess. The talks are scheduled to resume the week of August 29.