The US is reported to have put forward a package of incentives for North Korea to end its nuclear programme, during high-level talks in Beijing.
The talks are now expected to go on until Friday
The US has offered a written pledge not to attack if Pyongyang lets in nuclear inspectors, South Korea's Yonhap news agency quoted diplomats as saying.
South Korea's envoy confirmed a "concrete proposal" was on the table, but refused to give details.
US envoy Christopher Hill gave little away as talks entered their third day.
"At this point I don't want to say I am pessimistic or optimistic. I just don't know where we are going to end up or when we are going to end up," he said.
He described his meeting with his North Korean counterpart Kim Kye-gwan on Tuesday as business-like, with some "healthy exchange of views".
Treasury officials from both nations also met on Wednesday to discuss Pyongyang's demands that the US lift financial sanctions imposed last year.
The summit talks - which also involves China, South Korea, Japan and Russia - stalled 13 months ago after North Korea walked out in protest at the sanctions.
Pyongyang only agreed to return to the table after conducting its first nuclear test on 9 October this year.
South Korea's top negotiator, Chun Yung-woo confirmed the US had put forward a "formal, detailed and concrete proposal" to North Korea.
He did not elaborate on the details, but warned that both sides were still far apart.
"There has not been much narrowing of differences, but all the parties have laid the cards on the table and are discussing their priorities," he said.
The US proposal outlines a process towards the North's eventual nuclear disarmament, say unnamed diplomats, according to Yonhap.
Washington will give North Korea a written security guarantee - a pledge not to topple the regime by force - if it freezes its nuclear operations and allows UN inspectors to verify this, Yonhap reported.
Large-scale food, energy and other economic aid will follow if the North opens all its nuclear-related programmes for international inspection.
The US came to the talks hoping to revive an agreement reached with North Korea in September 2005, offering a similar package of incentives.
It also agreed to consider North Korea's demand that it lift its financial sanctions.
The US blacklisted a Macau-based bank containing $24m of North Korea's money in September 2005, accusing it of involvement in money-laundering and counterfeiting.
Daniel Glaser, the treasury official leading the discussions on the sanctions, appeared to rule out an imminent resolution, saying it "has to be a long-term process".
South Korea's Chun Yung-Woo said the six-party talks had been due to wrap up on Thursday, but would now continue through Friday because "substantive discussions" were being held.
Mr Hill has already said delegates should start working on a draft agreement if they hope to make any progress.
A South Korean parliamentary report on Tuesday suggested that divisions in North Korea over the successor to leader Kim Jong-il lay behind October's nuclear test.
Kim Jong-il agreed to the test to win military support for the eventual transfer of power to one of his sons, the report by the National Assembly's Intelligence Committee said.
"The fundamental task that North Korea has to address right now is not to overcome economic difficulties or establish diplomatic relations or a peace treaty with the US," the report said.
Its priority is "to stabilise the North's system through completing the designing" of the next leadership, it said.
The report also said the 64-year-old leader may begin the slow transfer of power to his son as early as next year. It did not say which of his three known sons it might be.