International efforts to rein in North Korea's atomic ambitions appear to be in trouble over what a Japanese delegate calls Pyongyang's "excessive" energy demands.

"The situation remains severe," Japanese envoy Kenichiro Sasae told reporters late Sunday, on the fourth day of six-nation talks in Beijing.

Sasae said talks on persuading the North to shut down its nuclear weapons program hit a snag after Pyongyang increased its demands for more energy aid. 

"The problem is that North Korea has excessive expectations about this, and unless it reconsiders this issue, an agreement will be difficult," he said.

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

Negotiators from the United States, China, Russia, Japan, and North and South Korea have agreed to reconvene on Monday, which would be the final day of talks.

U.S. Assistant Secretary of State Chris Hill, the American envoy, said he had hoped negotiators would reach a draft agreement and allow details to be worked out later.

"It looks unreasonable to expect a breakthrough today," South Korea's envoy, Chun Yung-Woo, told reporters as he left his hotel for the talks.

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.

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

The North's energy demands appeared also to have caused a schism among participants at the talks, 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 that the North demanded two 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.