Envoys were arriving in Beijing for the resumption later on Tuesday of six-nation talks on the issue. The latest round, the fourth since 2003, broke for a recess last month after a record 13 straight days of talks failed to yield an agreement.
North Korea's demands for a civilian nuclear programme are a sticking point, with Washington saying the North's record proves it cannot be trusted with any atomic project.
Chief North Korean negotiator Kim Kye Gwan said before he left for Beijing that his country will not tolerate any obstruction to its right to a peaceful nuclear programme, China's official Xinhua News Agency reported.
"This right is neither awarded nor needs to be approved by others," Kim said in Pyongyang. "If the United States tries to set obstacle to (North Korea's) using this right, we can utterly not accept that."
Still, Kim said the North would attend the talks with a sincere and flexible attitude, according to Xinhua.
Hill said little progress was made
in previous talks with N Korea
US Assistant Secretary of State Christopher Hill said on Monday upon arrival in Beijing that he was not sure how long the talks would last but would know more after contacts with the North Koreans.
No one-on-one meetings between the sides were planned, but Hill said he expected to speak with the North at a dinner for all delegates on Tuesday evening.
"I know that my delegation is coming here to work. We know pretty precisely what the issues are. I hope the (North Korean) delegation has also done some homework," Hill said.
US and North Korean diplomats met twice in New York in the past month, but Hill said there had been no progress on resolving the impasse beyond gaining an understanding of the North's position.
But he said "their position does seem to be evolving a little", without elaborating.
Last week in Washington, Hill reiterated a set of measures - including energy aid offered by South Korea - that he said would make it unnecessary for the North to pursue nuclear energy.
The North "has had trouble keeping peaceful programmes peaceful", he said on Friday.
South Korea's chief negotiator urged envoys to be open-minded at the talks.
"If each party can be a little more flexible in its position there will be good results, but if they stick to their current position, good results will be hard to expect," South Korean Deputy Foreign Minister Song Min-soon said as he arrived in Beijing.
"[The North] has had trouble keeping peaceful programmes peaceful"
US assistant secretary of state
Kenichiro Sasae, Japan's top envoy, said it was important first that the North "shows its determination in detail regarding dismantlement of their nuclear programmes".
"If this happens, I think it is possible that we could be more flexible in discussing the interests that North Korea has as the next step," he said.
China and Russia are also participating in the negotiations.