By Stephanie Nebehay
GENEVA (Reuters) - The United States is optimistic about an eventual return to six-party talks on North Korea's nuclear program, the chief U.S. negotiator said on Tuesday after two days of meetings with North Korean negotiators.
Ambassador Stephen Bosworth, U.S. 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.
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.
The talks on Tuesday were delayed at the request of North Korea, the U.S. diplomatic mission said earlier in a brief statement that declined to elaborate.
The morning session was canceled but the two delegations had a joint lunch at the DPRK mission followed by one-hour talks.
Bosworth was accompanied by Glyn Davies, the outgoing U.S. ambassador to the U.N. nuclear watchdog agency who has been named his successor, in the Geneva talks with North Korean First Vice Foreign Minister Kim Kye-gwan.
URANIUM ENRICHMENT KEY QUESTION
North Korean leader Kim Jong-il told Chinese Vice Premier Li Keqiang that a moribund 2005 deal should be the basis for new talks about Pyongyang's nuclear activity, Chinese state media said on Tuesday, leaving unanswered a key question on uranium enrichment, a possible pathway to atom bombs.
In his meeting with Li, Kim repeated that North Korea was willing to revive six-party talks that it abandoned after the United Nations imposed new sanctions for a long-range North Korean missile test. The following month, Pyongyang conducted a second nuclear test.
"Kim said the DPRK hopes the six-party talks should be restarted as soon as possible," said the Xinhua news agency report on Tuesday of the meeting between Kim and Li in North Korea on Monday night.
The North's uranium enrichment program, which opens a second route to developing an atom bomb along with its plutonium program, is not specifically referred to in the 2005 pact.
The North says that it is enriching uranium only for power generation and argues that the 2005 agreement respects its right to peaceful uses of nuclear energy.
(Additional reporting by Chris Buckley and Ben Blanchard in Beijing and Jeremy Laurence in Seoul; Editing by Michael Roddy)