(Reuters) - The United States is optimistic about an eventual return to six-party talks on North Korea's nuclear program but there were no breakthroughs during two days of meetings with North Korean negotiators, U.S. officials said on Tuesday.
Ambassador Stephen Bosworth, special representative for North Korea policy, said differences had been narrowed but did not reveal specifics of the talks with the senior North Korean negotiators.
The discussions had "touched on all issues," including humanitarian aid, but he declined to say whether North Korea's contested uranium enrichment program had been the focus.
"It has been a very useful meeting," Bosworth told reporters outside the U.S. diplomatic mission in Geneva.
"The tone was positive and generally constructive," he added.
"I am confident that with continued effort on both sides we can reach a reasonable basis of departure for formal negotiations for a return to the six-party process."
Washington and Pyongyang had a long history marked by "many differences," not all of which can be overcome quickly, he said.
"We narrowed differences on several points and explored our differences on other points. We came to the conclusion that we will need more time and more discussion to reach agreement," Bosworth said.
In the cautious world of U.S.-North Korean diplomacy, the comments were relatively upbeat. But U.S. State Department spokeswoman Victoria Nuland told reporters in Washington, "while there's been some narrowing of differences, we haven't had any breakthroughs here and significant issues do remain."
The two sides held bilateral talks in New York in late July, the first since six-party talks over Pyongyang's nuclear weapons program collapsed in 2009. The wider talks include South Korea, China, Japan and Russia.
The United States and South Korea insist that the North immediately halt its uranium enrichment work, which it unveiled last year, as a precursor to restarting regional talks that offer economic aid in return for denuclearization by Pyongyang.
U.S. and North Korean officials would consult with their respective capitals and stay in touch via the New York diplomatic mission of the Democratic Republic of Korea (DPRK), according to Bosworth.
There was no specific timetable for the next round of talks, a U.S. official told Reuters.