Talks on North Korea's nuclear ambitions appear to have stalled, delegates meeting in Beijing have said.
The US negotiator says North Korea's request is a big problem
The chief US negotiator said the talks were at a "standoff", while a North Korean spokesman blamed the US for the lack of progress.
The impasse stems from a new North Korean demand that it be provided with a light-water nuclear reactor as part of a deal to give up nuclear weapons.
The six-nation discussions reconvened on Tuesday after a five-week recess.
North Korean spokesman Hyun Hak-bon said Pyongyang wanted to be given a light water reactor to make up for the graphite moderated reactors it was being pressed to give up under the deal.
N Korea is being pressed to give up nuclear weapons and close Yongbyon reactor
In return it will receive security guarantees, economic aid and free electricity
N Korea first said it wanted right to maintain civilian nuclear programme
Now says it wants to be given a light water reactor
N Korea was promised two reactors under 1994 deal, but deal broke down in 2002
Light water reactors are much more difficult to use as a source of plutonium with which to build nuclear weapons. North Korea claims to be using one of its existing graphite reactors to this end.
"While other participating countries expressed understanding on this issue, the United States unreasonably vowed not to provide light water reactors," Mr Hyun said.
The statement appeared to open up a fresh divide between North Korea and the US.
Christopher Hill, the US' chief negotiator, has called the North Korean request a "major problem" for the talks, though hosts China said they would continue.
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
Mr Hill said no progress had been made on Thursday.
Mr Hill has said that building a light water reactor would cost $2-3bn (£1.1-1.6bn) and would take about 10 years, and insisted the North accept a South Korean offer to use its electricity through power cables across the border.
Mr Hill said agreement needed to be reached on the removal of North Korea's "terrible weapons" before other demands could be looked at.
Pyongyang, for its part, wants aid and diplomatic incentives and be granted the right to keep civilian programmes first.
South Korea has offered to supply the North with 2,000 megawatts of electricity - roughly the North's current total power output.
But the South Korean chief envoy, Song Min-soon, said on Wednesday that once the dismantlement of nuclear weapons and programmes was completed, Pyongyang should be granted "the right to peaceful use of nuclear energy".
The six-party nuclear talks involve North Korea, the US, China, Japan, Russia and South Korea.