For the last couple of days, envoys to the six nation talks on North Korea's nuclear weapons programs have said "a single issue" stands in the way of a preliminary deal.
On Sunday, senior Japanese negotiator Kenichiro Sasae revealed what that issue was. Sasae told reporters North Korea's "excessive demands" for energy compensation were the main obstacle to an agreement.
Senior South Korean negotiator Chun Yung-woo says it would be "unreasonable" to expect a breakthrough on Sunday. He agrees with Sasae that the primary dispute is over how and when to compensate North Korea.
He says the main problem is how to tie any economic or energy aid to the speed and scope of North Korea's steps toward nuclear dismantlement.
The negotiators have been working from a proposed list of reciprocal steps, drafted earlier in the week by China, that would start the nuclear disarmament process.
The ultimate aim is implementation of North Korea's September 2005 pledge to scrap its nuclear weapons programs in return for economic and energy aid, security guarantees and diplomatic benefits.
The chief U.S. negotiator, Assistant Secretary of State Christopher Hill, said Sunday it was time, in his words, to "wrap this up and get moving."
Although Hill has not specifically identified energy as the issue delaying the talks, he does say North Korean demands are what is holding up a deal.
"I would say the problem is North Korea wanting a very precise measurement of how we're going to go forward."
In a 1994 agreement between North Korea and the United States, Washington agreed to provide two so-called light water nuclear reactors to the North in exchange for Pyongyang shutting down its existing, older-style nuclear reactor at Yongbyon.
That deal failed after Washington confronted North Korean authorities in 2002 with evidence that Pyongyang was violating the agreement. North Korea restarted the reactor in 2003, and experts believe it has now produced enough plutonium to make several nuclear bombs. Last October, the North conducted its first nuclear test.
Hill has called for patience over the past two days, and his public comments have been less pessimistic than those of his Japanese and South Korean colleagues. He says a separate dispute with Pyongyang over U.S. financial sanctions targeting alleged North Korean illegal activities is no longer a hindrance to the nuclear talks.
China and Russia are also party to the talks, which are now in their fourth day.