International talks to persuade North Korea to dismantle its nuclear program verged on foundering Sunday over the country's demands for energy aid, and would end after one more day of negotiations, envoys said.

A day of meetings and an unscheduled nighttime session by the negotiators from the United States, China, North Korea and three other countries failed to achieve much progress, envoys from Japan and South Korea said. The six envoys agreed to reconvene Monday.

U.S. Assistant Secretary of State Chris Hill said the talks had snagged on the amount of energy assistance Pyongyang is to be awarded as an inducement for disarming. But he said that a deal could still be reached Monday, which would be the final day of talks.

"The situation remains severe," Japanese envoy Kenichiro Sasae told reporters late Sunday. He added that North Korea offered no new proposals.

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South Korean envoy Chun Yung-woo said the sticking points touch on vital interests of many of the parties. "It's not a situation where a breakthrough is in sight," Chun said.

Negotiators had hoped that the talks would concrete steps by North Korea to dismantle its nuclear programs in more than three years of negotiations and especially since Pyongyang's successful nuclear test in October.

Pressure for a breakthrough was high, in part because the United States has said it would be pointless to continue the often inconclusive negotiations without progress.

"Because this round could be the crossroads, today's talks took place in an atmosphere of heightened tension," Sasae said.

Envoys have shown rising frustration at North Korea's intransigence, repeatedly saying negotiations were getting bogged down over a single issue. Sasae and Russian Deputy Foreign Minister Alexander Losyukov have said that energy assistance was the sole sticking point, with the Japanese envoy calling North Korean demands "excessive."

"This is the problem, and unless they change their thinking, an agreement will be difficult," Sasae said earlier Sunday.

In a further sign of the difficulties, Losyukov lowered expectations of a breakthrough, saying consensus on how much energy to provide North Korea may be unattainable.

"I don't think that we can resolve the issue of specific numbers or specific amounts during these talks," Losyukov said after a meeting with South Korea's Chun.

It was unclear how much energy the North was demanding, with reports varying from 2 million kilowatts of electricity — an amount equal to all of North Korea's current generating capacity — to 2 million tons of heavy fuel oil.

But the North's demand appeared also to have caused a schism among participants, with Japan refusing to chip in until the issue of its citizens abducted by North Korea is resolved.

Chun stressed the burden of supplying energy aid should be shared.

"Even if we want to do it alone, other countries won't allow us to do that," he said. Chun also denied the North demanded 2 million kilowatts of electricity.

A rough outline of a deal was reached 18 months ago: In return for giving up its nuclear programs, North Korea would receive energy assistance for its listless economy and guarantees that its security would not be undermined.

Hill said his North Korean counterpart Kim Kye Gwan had told him at their Sunday meeting that he was "taking some ideas and thinking about them" to work around the problem.

This round of talks started amid optimism after the North Korean and U.S. envoys held an unusual meeting in Germany last month and reportedly reached an understanding.

A newspaper linked to the Pyongyang government said Sunday that the U.S. had agreed at those talks to lift restrictions on a Macau bank where the North's government had accounts within 30 days — the issue that deadlocked the last round of nuclear talks. In return, the North would take first steps to disarm in 60 days, the Japan-based Choson Sinbo said, citing an unnamed source.

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