"The fundamental thing is to make real progress in realising the denuclearisation of the Korean Peninsula," North Korean Vice Foreign Minister Kim Kye Gwan said in a statement at the opening session of the talks in Beijing on Tuesday.
The negotiations are aimed at convincing North Korea to abandon its nuclear programme.
"This requires very firm political will and a strategic decision of the parties concerned that have interests in ending the threat of nuclear war," he said. "We are fully ready and prepared for that."
The talks on Tuesday are the fourth such six-nation negotiations, which also include China, Japan, Russia, South Korea and the United States. They are reconvening after a 13-month boycott during which Pyongyang refused to attend, citing "hostile" US policies.
North Korea agreed to return to the talks after a meeting earlier this month between Kim and the main US envoy, Assistant Secretary of State Christopher Hill, who reassured the North that Washington recognised its sovereignty.
On Tuesday, Hill repeated his assurances. "We view [North Korea's] sovereignty as a matter of fact. The United States has absolutely no intention to invade or attack" it, Hill said in his opening remarks.
Unlike the previous rounds, no end date has been set for the resumed negotiations. Hill on Tuesday said his delegation would remain in Beijing "so long as we are making progress in these talks".
"Nuclear weapons will not make [North Korea] more secure. And in fact, on the contrary, nuclear weapons on the Korean Peninsula will only increase tension in the region"
US assistant secretary of state
The US diplomat said the talks were at a "critical juncture" and promised that the US delegation was ready for "serious negotiations".
"We do not have the option of walking away from this problem," he said.
"Nuclear weapons will not make [North Korea] more secure," he added. "And in fact, on the contrary, nuclear weapons on the Korean Peninsula will only increase tension in the region."
Neither the North Koreans nor the Americans offered any new proposals or concessions in their opening comments.
South Korean offer
South Korea's envoy, Deputy Foreign Minister Song Min-soon, repeated his nation's offer of massive electricity aid to the North if it agreed to disarm.
The meeting commenced with an appeal by host China for flexibility at the table.
While few expect a breakthrough, the atmosphere in the run-up to the talks has been upbeat.
Seoul's Song Min-soon promises
electricity aid if the North disarms
"I hope relevant sides ... can take a flexible and pragmatic attitude in the talks, respect each other, engage in dialogue on an equal basis and have full consultations," Chinese Foreign Minister Li Zhaoxing said as the talks opened.
Japan's top negotiator Kenichiro Sasae warned that failure to gain concrete results at the latest round would call the credibility of the talks into question.
The latest nuclear standoff with North Korea erupted in late 2002, when US officials accused the communist nation of running a secret uranium enrichment programme.
Since then, the North has pulled out of the Nuclear Non-proliferation Treaty and taken steps that would allow it to harvest more radioactive materials for atomic bombs.
In February, Pyongyang publicly said it had nuclear weapons, but it has not performed any known tests that would confirm it can make them.