A new round of international talks aimed at ending North Korea's nuclear programme is getting under way in the Chinese capital, Beijing.
North Korea has already shut down its main reactor at Yongbyon
Envoys from the six countries involved in the talks will discuss exactly when and how Pyongyang will disclose and dismantle all its nuclear facilities.
North Korea tested a nuclear device last year, but then agreed to end its nuclear programme in return for aid.
It has already shut down its main nuclear reactor at Yongbyon.
The six-party talks involve the two Koreas, China, Japan, Russia and the United States.
At talks in Geneva earlier this month, North Korea agreed to disclose and dismantle its nuclear facilities by the end of the year, US officials said.
N Korea to "shut down and seal" Yongbyon reactor, then disable all nuclear facilities
In return, will be given 1m tons of heavy fuel oil
N Korea to invite IAEA back to monitor deal
Under earlier 2005 deal, N Korea agreed to end nuclear programme and return to non-proliferation treaty
N Korea's demand for light water reactor to be discussed at "appropriate time"
Chun Yung-woo, South Korea's chief delegate, said he anticipated tough negotiations on the details.
"Because the disablement and declaration phase is a road no one has ever walked on, setting a guideline and a milestone is going to be... difficult and important," he said.
Ahead of the talks, US chief negotiator Christopher Hill met his North Korean counterpart, Kim Kye-gwan.
Mr Hill said that the two sides appeared "very much on the same wave-length", but that more detailed talks were necessary.
Mr Kim, meanwhile, said that all sides would try "to produce the outcome and create some rapport in this six-party talks, not to discourage everyone".
But recent concerns over alleged North Korean nuclear cooperation with Syria have threatened to undermine optimism over the talks.
North Korea has denied allegations that it may be helping Syria develop a nuclear weapons facility.
The allegations emerged following reports earlier this month that Israeli jets entered Syrian air space and hit an unknown target.
Mr Hill said that he had raised the issue of proliferation with Mr Kim.
"I think I've made it very clear that the issue of proliferation, generally, has always been an issue and it continues to be," Yonhap news agency quoted him as saying.
The talks are due to continue until Sunday.