China has said international talks aimed at persuading North Korea to abandon its nuclear programme are to go into recess until the end of August.
US delegate Christopher Hill blamed the deadlock on North Korea
It comes after delegates from the six nations involved remained deadlocked after a 13th day of negotiations.
North Korea has blamed the deadlock on the US. "We had to produce nuclear weapons because the US is threatening us with nuclear weapons," it said.
The US said North Korea's demand to use light-water reactors was the obstacle.
'Not on the table'
The two Koreas, the US, China, Japan and Russia have been holding talks in China's capital, Beijing, since 26 July.
The US wants North Korea to abandon its nuclear weapons in return for aid and security guarantees.
But North Korea insists it has the right to conduct nuclear activities as long as they are peaceful - for example, for generating electricity.
Speaking after the talks broke up, the chief US negotiator, Assistant Secretary of State Christopher Hill, said North Korea's insistence on being allowed to have light-water reactors for energy purposes had prevented an agreement.
Oct 2002: US says North Korea is enriching uranium in violation of agreements
Dec 2002: North Korea removes UN seals from Yongbyon nuclear reactor, expels inspectors
Feb 2003: IAEA refers North Korea to UN Security Council
Aug 2003: First round of six-nation talks begins in Beijing
Feb 2005: Pyongyang says it has built nuclear weapons for self-defence
He said: "The issue came down to the DPRK [North Korea]. They not only want the right to use nuclear energy, but the right to use light-water reactors. That is simply not on the table."
Light-water reactors are capable of producing weapons-grade nuclear material.
Kim Kye-gwan, the chief North Korean delegate, blamed the US refusal to allow his country to maintain a peaceful nuclear programme for the deadlock.
The disagreement over "peaceful nuclear activity" was "one of the very important elements that led us to fail to come up with an agreement", he said.
Wu Dawei, Chinese chief negotiator and chairman of the talks, said they would resume in the week of 29 August, after the delegates had had a chance to return home for consultations.
He described the negotiations so far as useful and said those involved had "reached agreement in many aspects" .
"The six parties, in the spirit of mutual respect and equality, held serious, practical and in-depth discussions and consultations in a good atmosphere on the goal of the denuclearisation of the Korean peninsula, which led to better mutual understanding, broader common ground and positive progress," he said.
The six nations have struggled to agree on a joint statement that would provide for the dismantling of North Korea's nuclear programmes.
Mr Wu said he did not know when agreement would be reached but he believed it would happen "one day".
The US wants all the North's nuclear facilities dismantled before any concessions are made, and has said it is not prepared to compromise on the issue.
The nuclear crisis first erupted in 2002, when the US accused North Korea of pursuing an uranium enrichment project to make nuclear weapons.
The stand-off deepened when Pyongyang withdrew from the Non-Proliferation Treaty (NPT), and tensions were raised further when it announced earlier this year that it had plutonium-based nuclear weapons.
The North continues to deny US reports of a uranium-based capability.
Three previous rounds of talks have ended in failure, but this fourth round has gone on for a much longer time, in what analysts saw as a sign that all sides seemed determined to find a solution.