The US, N Korea, Russia, China, South Korea and Japan are represented
Senior diplomats are meeting in Beijing to thrash out the next move in the long-running mission to end North Korea's nuclear ambitions.
US envoy Christopher Hill said talks would seek to agree practical steps to verify North Korea's account of its nuclear programme, provided last month.
The talks between delegates from both Koreas, Russia, the US, China and Japan are resuming after a nine-month delay.
Our correspondent in Seoul, John Sudworth, says they are at a key stage.
There are still some major potential stumbling blocks in the way of the long-term goal of completely dismantling all of North Korea's nuclear programmes, he says.
The mood among the envoys is mixed, with China's Wu Dawei emphasising the parties' shared objective - "the realisation of a nuclear-free Korean peninsula" - while South Korea's Kim Sook cautioned that it was a "very difficult process".
'Weeks or months'
The meeting in Beijing will focus on ways to verify the contents of the declaration, by seeking agreement for international inspectors to visit nuclear sites, inspect equipment and interview officials.
NORTH KOREA NUCLEAR DEAL
2002: N Korea pulls out of previous deal after US accuses it of having secret uranium programme
October 2006: North Korea carries out its first test of a nuclear weapon
February 2007: N Korea agrees to end nuclear activities in return for aid
July 2007: North Korea closes its Yongbyon nuclear reactor and allows IAEA inspectors in
December 2007: N Korea misses a deadline to hand over a declaration of its nuclear work
June 2008: N Korea hands over nuclear programme details; US cautiously welcomes the move
North Korea missed a December 2007 deadline to provide the nuclear declaration, but finally submitted it last month.
It is in the process of removing the fuel rods from its Yongbyon nuclear reactor in return for economic aid - including the equivalent of a million tonnes of fuel.
It is likely to seek to speed up provision of this aid at the talks.
Meanwhile, Mr Hill told reporters the other parties were hoping "to produce a verification regime that will lay out the rules for the road".
He said the verification process would take "longer than just a few days... several weeks or even months. But we need to agree on how verification will work".
Analysts credit the pragmatic Mr Hill with providing much of the driving force behind the negotiation process.
But conservative critics of the Bush administration's approach to North Korea - which has undergone a major shift since North Korea was declared a part of an global "axis of evil" - complain it is handing over too many concessions, too quickly.
The US says there are outstanding questions relating to the North's suspected uranium enrichment programme and proliferation activities.
Pyongyang agreed to scrap its nuclear ambitions early last year. In return, the other five nations involved with the negotiations agreed to provide fuel and diplomatic concessions.
But the road to denuclearisation has been far from smooth, and the six-party talks foundered in the wake of Pyongyang's failure to hand over a declaration of its programme last December.
The regime unexpectedly produced the long-awaited document in late June.
It then took the symbolic step of blowing up the already decommissioned cooling tower at its Yongbyon reactor.
The US responded by agreeing to scrap some of the sanctions it had imposed on the regime. It has also begun steps to remove the North from a list of state sponsors of terrorism.
Last week, the North warned it would take no further steps to dismantle its nuclear facilities until the six-nation group provided the fuel and political benefits it had been promised.
The regime's foreign ministry said in a statement it that has disabled 80% of its main nuclear complex, but has been supplied with only 40% of promised energy shipments.
"We stick to the principled basic position that the principle of action for action must be respected," AFP news agency quoted a North Korean foreign ministry spokesman as saying on 4 July.