The last round of talks ended with North Korea promising to disarm in exchange for aid and a security guarantee. But negotiators have not taken up the most difficult issues: how the North will disarm and how to verify it.
North Korea has raised doubts about its intentions by demanding it be given a civilian nuclear reactor before it disarms, a condition the United States has rejected.
The joint statement last month sidestepped the North's demand for the reactor, saying it would be discussed "at an appropriate time."
Earlier Tuesday, Kim Gye Gwan said North Korea "cherishes the joint statement," China's official Xinhua News Agency reported.
"We are willing to make sincere efforts at this round of the talks to fulfill the spirit of the joint statement," Kim was quoted as saying in Pyongyang before flying to Beijing.
Analysts cautioned against expecting a breakthrough.
Pyongyang appears to be dragging its feet, said Peter Beck, the Seoul-based director of the North East Asia Project for the International Crisis Group, an independent think tank.
"I don't think they're serious about progress yet," he said. In the meantime, he said, "Washington has no choice but to go along with this charade."
Even host China tried to lower expectations, saying this week's meeting could be considered a success even if it produces no written agreement.
"I do not think that progress of the talks needs to be measured by the signing of a document," Foreign Ministry spokesman Liu Jianchao said. "During the process, all parties will enhance their understanding for each other and accumulate consensus."
In Washington, Siegfried Hecker, a U.S. scientist who toured North Korea's reactors in August, said he believes Pyongyang is "moving full speed ahead with its nuclear weapons programs."
Hecker, a senior fellow at the Los Alamos National Laboratory, was given an inside look at apparent plutonium production by North Korean scientists.
"They're poised to continue their program, to make more plutonium and to strengthen their deterrents," Hecker said at a nuclear nonproliferation conference in Washington. "We have to assume that the North Koreans also have made at least a few primitive nuclear devices."
U.S. intelligence has previously estimated that North Korea has separated enough plutonium for at least one or two nuclear weapons.
South Korean President Roh Moo-hyun said the talks were the only way to resolve the dispute, which erupted in 2002.
"Although it may take some time," Roh said at a luncheon with foreign journalists in Seoul, "failure is inconceivable."
