Beijing ? An improved atmosphere might be the most significant accomplishment as six-nation talks on North Korean nuclear disarmament stretched into their longest round Friday, but the top U.S. envoy stressed “this isn’t going to be easy.”

After a fourth session of one-on-one meetings, American and North Korean diplomats remained split over the North’s demand for U.S. concessions before giving up its nuclear weapons program and its insistence on having a peaceful atomic energy project.

Nevertheless, “I think all would agree that we have a continuing good atmosphere,” State Department spokesman Sean McCormack said.

Delegates were working on a statement of principles that could evolve into an agreement, McCormack said in Washington. “You have all the parties agreeing what the goal of the six-party talks is now: a denuclearized Korean peninsula,” he said.

U.S. Assistant Secretary of State Christopher Hill’s meetings with North Korean Vice Foreign Minister Kim Kye Gwan have been a marked change that has raised optimism over the talks, which have been run more flexibly than the previous rigidly scheduled negotiations.

“The fact that they’re continuing to talk to each other is by far the most encouraging sign,” said Peter Beck, the Seoul-based director of the North East Asia Project for the International Crisis Group, an independent think tank.

Beck said the latest round of talks has continued longer than previous rounds – which were marked by bombast – because neither the North nor the Americans seemed to want to be blamed for scuttling the discussions by walking away.

Hill said the sides also hadn’t come to terms on North Korea’s alleged uranium enrichment program or whether Pyongyang should be allowed to have a peaceful atomic energy project.

“Still we have a lot of differences that remain,” Hill told reporters Friday evening. “I don’t want to suggest for a minute that this is going to be easy.”

Despite the apparent impasse, the No. 2 South Korean delegate, Cho Tae-yong, said Friday’s meetings “were not lower than my expectation.”