China has warned "major" differences remain to be resolved in multilateral talks aimed at dismantling North Korea's nuclear weapons programme.
North Korea said it would be patient and carry on with the talks
It came on the second day of low-level talks in Beijing involving both Koreas, the US, China, Russia and Japan.
North Korea described as "humiliating" Washington's insistence on a complete dismantling of its programme.
But an official said Pyongyang, seeking US assurances on aid and security, would be patient with negotiations.
The crisis began in October 2002, when the US said North Korea was working on a secret uranium enrichment programme.
"There still exist differences between each side in some areas, and sometimes the differences are major," said Chinese foreign ministry spokesman Liu Jianchao.
He said there had been several "candid and frank" talks, with new "content" emerging, although he did not elaborate.
According to reports, the working-group discussions began on Wednesday with the US and North Korea showing little willingness to compromise.
"Their positions look even tougher now than a few days ago," an unnamed diplomat told Russia's Itar-Tass news agency.
The talks come after two rounds of high-level meetings failed to yield any major progress.
The meetings are meant to help prepare for a third round of six-nation talks, expected to take place in Beijing before the end of June.
What goes on in North Korea's nuclear facilities is unclear
But few expected much progress.
Some analysts predicted that Washington was unlikely to make any compromises ahead of the presidential election in November.
Early Friday, a North Korean official read a statement criticising Washington for its tough stance on a "complete, verifiable and irreversible dismantling" (CVID) of its nuclear programme.
"The US demand on CVID is the kind of humiliating measure that can only be imposed on a country defeated in a war," said the official, who identified himself as Pak Myong-Kuk, one of the negotiation team.
However, he said Pyongyang "expressed its willingness to maintain patience and proceed with the six-party process with patience".
North and South Korea are also planning to hold their first general-level military talks for over 50 years, in a bid to ease tensions in the region.
The defence ministers of the two Koreas have met before, as have junior military officers.
But generals - the most senior working military officers - have not held talks since the end of the Korean war in 1953.
Defence officials in Seoul said the talks - which North Korea will host - are to take place on 26 May.