With the six nations at the talks meeting on Friday, the fourth day of the current round of talks, the crux of the disagreement is over timing: whether Pyongyang should receive security guarantees and aid before it moves to scrap its weapons programmes, as it insists, or if it should move first, as Washington wants.
"It's not a matter of who goes first; it's a matter of a strategic commitment that the goal of a nuclear-free Korean Peninsula is embraced by all," US Secretary of State Condoleezza Rice said in her first comments on the current round of negotiations.
"We know that the South Koreans, the Japanese, the Russians, the United States and China all agree that it has to be a non-nuclear Korean peninsula. The question is do the North Koreans embrace that goal as well?," Rice asked in Washington.
The North has also demanded Washington remove nuclear weapons from the peninsula. The US, which keeps more than 30,000 troops in South Korea, says it no longer has such weapons there.
The negotiations, which resumed this week after a 13-month hiatus, have seen the parties retreat to familiar territory, with North Korea demanding aid and security guarantees before scrapping its nuclear programmes and the US insisting it scuttle those programmes first.
"Although the United States and North Korea have had in-depth discussions, my understanding is that they have not narrowed their differences to the extent that they can claim progress," a Japanese delegate said.
Still, the pattern of unusually frequent and lengthy one-on-one exchanges on the sidelines of the talks - Thursday's lasted three hours - signals a shift in the US approach and has raised hopes for a positive outcome.
"It's far too early to say if it's a breakthrough or a breakdown"
Chinese delegation spokesman
Chinese delegation spokesman Qin Gan said on Thursday that talks were moving in the right direction but added: "It's far too early to say if it's a breakthrough or a breakdown."
No end date has been set for the talks. The parties were holding bilateral meetings on Friday morning and all six - both Koreas, the United States, Japan, Russia and host China - were set to meet in the afternoon.
The nuclear standoff erupted in October 2002 when US officials accused Pyongyang of pursuing a clandestine weapons programme, prompting it to expel UN nuclear inspectors.
In February, North Korea announced that it had nuclear weapons and demanded Washington provide aid, security guarantees and diplomatic recognition in return for scrapping them.