The second phase would see North Korea permanently disabling the Yongbyon complex as well as declaring and dismantling all other nuclear weapons-related activities.
In return North Korea has been promised a million tonnes of heavy fuel oil, as well as other political, diplomatic and economic benefits.
The talks had been scheduled to end on Thursday, but were extended by an extra day in an effort to reach consensus on the next phase.
"I really think that North Korea has taken a great positive step... Bravo North Korea"
andrew, Kampala, Uganda
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The talks bring together envoys from North and South Korea, China, Russia, Japan and the US.
"I'm still of the view that with a little luck we can wrap this all up by the end of the year, but obviously it's going to be difficult," Christopher Hill, the chief US envoy to the talks, told reporters before leaving Beijing.
He said the important issue was not an exact month, but
"that we continue to make perceptible progress".
Hill said earlier he continued to believe that it was "quite doable" for North Korea to complete all phases of disarmament by the end of the year.
On Wednesday the International Atomic Energy Agency said North Korea had shut five main nuclear facilities at Yongbyon, completing the first stage of a disarmament deal reached in February.
The facilities include a reactor and an atomic fuel reprocessing plant that can extract the plutonium that was used in the North's first nuclear test explosion last year.
|Hill said the talks had built a |
momentum for future progress [EPA]
Despite the failure to agree on a deadline for the second phase, Hill said this week's talks had been extremely positive and had "built momentum" for future progress.
"Look how far we have come in the last seven days," he said, referring also to the shutdown of the Yongbyon reactor complex.
Chun Yung-Woo, South Korea's lead envoy to the talks, agreed that the latest round had been a success, saying North Korea had "clearly expressed its intention not to delay the implementation of the February 13 agreement".
"North Korea has said it intends to fulfil its obligations, if conditions are met, as quickly as possible under the agreement."
Hill and Chun said a final deadline could still be set after all parties broke into working groups to hammer out the details of what needs to be declared and how to close them.
The working groups are expected to meet over the next few weeks.
Wu Dawei, the Chinese envoy to the six-party talks, said a fresh round of negotiations would take place in September to "work out the road map" for disarming North Korea.
Wu added that the North remained committed to winding down its nuclear activities.