China has warned that solving the North Korean nuclear crisis will be a "long, drawn-out process", as it prepared to host six nation talks on the issue.
Talks will focus on the Yongbyon nuclear site
The talks, the first in six months, are likely to explore North Korea's offer to freeze its nuclear programme in return for concessions.
But it is not clear if the US and North Korea will be able to end disagreement over the timing of those concessions.
A row over the North's alleged uranium programme could also prevent progress.
Chinese Foreign Ministry spokeswoman Zhang Qiyue said the nuclear crisis was "complicated", but that Beijing hoped consensus could be reached on at least some of the issues.
North Korean Deputy Foreign Minister Kim Kye-gwan was quoted by China's Xinhua news agency as saying the talks would be "difficult", but that the context was more promising than that of the last six-party negotiations in August.
Due to open 0900 (0100 GMT) Wednesday
China, US, Russia, Japan, North and South Korea taking part
Set to last until Friday
Parties to sit at hexagonal table, US next to N Korea
China reported that Pyongyang had repeated its offer to freeze its nuclear facilities as a step towards the total abolition of its nuclear programme. But it was not clear whether the offer included the alleged uranium programme, which North Korea has publicly denied exists.
Diplomats said another potential sticking point was the mechanics of such an offer. Pyongyang has always insisted that a freeze must be met with simultaneous US concessions - economic and diplomatic rewards - but the US wants North Korea to move first.
"If anyone proposes at the upcoming six-party talks that compensation be proceeded (sic) by a freeze, the DPRK [North Korea] will resolutely oppose it," the North Korean Foreign Ministry said on Tuesday according to Xinhua.
A BBC correspondent in Beijing, Charles Scanlon, says that some diplomats have warned of a complete breakdown in dialogue if the United States and North Korea stick to their long-held positions.
Another issue which threatens to divide the delegates is that of the kidnapping of Japanese civilians by Pyongyang during the 1970s and 1980s. North Korea says this should not be raised during the talks, even though Tokyo has said it plans to do so.
Step by step approach
South Korea has proposed that the North freeze its nuclear programme as part of a three-step procedure to scrapping it altogether, according to South Korea's delegate, Deputy Foreign Minister Lee Soo-hyuck.
The steps are reported to be:
Phase 1: North Korea states its readiness to dismantle its nuclear programmes, in return for which, the US states its readiness to provide security guarantees for North Korea
Phase 2: North Korea freezes its nuclear programmes. This, once verified, earns North Korea energy aid and other rewards
Phase 3: The verified dismantling of all North Korea's nuclear facilities and the resolution of all related issues
North Korea has agreed before to halt activities at its plutonium generator - at Yongbyon, 90 kilometres (50 miles) north of the capital, Pyongyang.
But the 1994 deal with the United States broke down in late 2002, and since then North Korea has claimed to have finished reprocessing 8,000 spent fuel rods stored at Yongbyon - enough to help it build up to six more nuclear weapons.
Our correspondent says that despite the obstacles, all sides have been working hard to keep the talks on track.
North Korea badly needs economic help for its wrecked economy and the Bush administration is unlikely to welcome another crisis over weapons of mass destruction in the run up to November's election.