IsolationHill would not specify the dispute, but said it concerned one paragraph of a draft deal and said failure to resolve it would augur badly for the six-party talks, which resumed in December after a hiatus from late 2005.
"We are kind of reaching a point where we've got to solve this, and if we don't solve this I think it's sort of tough to reconvene the six parties," he said.
A diplomatic source close to the talks said the dispute concerned the volume of energy and economic aid to North Korea, whose economy has struggled in isolation.
"A huge gap remains between North Korea and the five countries in terms of figures and volume," the source said.
Negotiators still hoped to agree on a joint statement spelling out Pyongyang's first disarmament steps and it was worth staying in China and trying to clinch the deal, Hill said.
He also said the talks could go another day or two, if not longer.
"Tension boiled after Pyongyang staged its first nuclear test blast last October and the UN responded with sanctions"
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North Korea is under pressure to accept a deal, not least from China, its communist neighbour and backer lately angered by Pyongyang's nuclear brinksmanship.
Some negotiators have said the proposed initial deal will demand North Korea shut down its Yongbyon nuclear plant, which produces plutonium that can be used in nuclear weapons.
Chun Yung-woo, South Korean envoy, said the parties' interests were not meshing easily, though the North had a firm position on how it would denuclearise.
He said: "As they like to say these days, we can't predict when the egg will hatch, but we know for sure that it's not an unfertilised egg."
The fresh momentum among the six sides came after US and North Korean teams met for unprecedented talks in Berlin last month.
That meeting cooled tension after Pyongyang's first nuclear test in October, which triggered UN sanctions.
Japan's Kyodo news agency said North Korea had demanded energy aid equivalent to more than 2 million tonnes of fuel oil annually in exchange for initial steps toward abandoning its nuclear programmes.
Kyodo also quoted Alexander Losyukov, the Russian envoy, as saying assistance and the timing for when it should be given to North Korea are "the most difficult problems".