Envoys at talks on North Korea's nuclear program struggled early Tuesday to salvage a last-minute agreement for Pyongyang to take its first steps toward disarming in more than three years of negotiations.

A marathon session that began Monday morning stretched more than 14 hours as delegates huddled at a Chinese state guesthouse past midnight, trying to resolve a dispute over how much energy assistance the North would receive for giving up its atomic weapons.

"It is up to the North Koreans. We have put everything on the table. We have offered a way forward on a number of issues. They just need to make a decision," U.S. Assistant Secretary of State Christopher Hill told reporters early Monday before meetings that he said would be the last day of talks.

A South Korean official said late Monday that negotiations on an agreement were headed toward a possible conclusion, but that any accord would still require approval from the countries' governments.

The negotiations have "reached a final phase," the official said on condition of anonymity due to the ongoing diplomacy.

Chinese envoy Wu Dawei told a visiting Japanese lawmaker that North Korea had agreed to shut down its main nuclear reactor and submit a list of its atomic facilities. But the size of the energy aid Pyongyang would get in return was still undecided, the lawmaker, Fukushiro Nukaga, told reporters Monday.

Monitor the Korean Peninsula nuclear showdown in FOXNews.com's North Korea Center.

The current round of six-nation talks began Thursday on a promising note after the United States and North Korea signaled a willingness to compromise. But negotiations quickly became mired on the energy issue.

The negotiations — which include the two Koreas, the U.S., Japan, China and Russia — have plodded on intermittently since 2003.

Adding pressure on the delegates was a sense that failure to reach an agreement this time could permanently doom the talks.

"There's a certain life cycle to these negotiations," Hill said Monday. If North Korea rejects the current proposal, the American diplomat speculated that there would "be some political climate change, if not in the U.S., then maybe among some other countries."

But he added, "I don't want to predict that this is the last chance."

Negotiators had hoped the latest round would result in North Korea taking its first concrete steps in dismantling its nuclear program, an issue that became especially critical after it conducted its first nuclear test explosion in October.

South Korean and Japanese media reports gave varying accounts of how much energy North Korea was demanding, including up to 2 million kilowatts of electricity or 2 million tons of heavy fuel oil.

Under a 1994 U.S.-North Korea disarmament agreement, the North was to receive 500,000 tons of fuel oil a year before construction was completed of two nuclear reactors that would be able to generate 2 million kilowatts of electricity.

That deal fell apart in late 2002 when the U.S. accused the North of conducting a secret uranium enrichment program, sparking the latest nuclear crisis.

Complete coverage is available in FOXNews.com's North Korea Center.