South Korea's top nuclear envoy is on his way to Russia amid accelerating efforts to set terms for a resumption of six-party talks on North Korea's nuclear programs.

South Korean officials said Lim Sung-nam left Wednesday for Moscow to discuss the latest progress with his Russian counterpart, Alexei Borodavkin.

The visit follows what were described as “positive” and “generally constructive” talks in Geneva Tuesday between the U.S. and North Korean negotiators.

The U.S. envoy Stephen Bosworth said afterward he was confident that, with continued effort, the sides will be able to find a reasonable basis for a return to the six-party talks. But other U.S. officials said it may be weeks or months before North Korea makes its intentions clear.

Lim told South Korea's Yonhap news agency before departing that he and Borodavkin will review the progress to date and discuss future moves.

The six-party talks have been stalled since North Korea walked out in 2009, but in recent months North Korea has been pressing to have them resume. Since July, Pyongyang's nuclear envoys have met twice each with the United States and South Korea to discuss possible terms.

North Korea has called for the talks to resume without preconditions, but the United States and South Korea are demanding that Pyongyang honor some of its previous commitments as a show of sincerity. They also insist that North Korea halt a uranium enrichment program that was revealed since the six-party talks broke down.

Bosworth said the differences had been narrowed but not resolved during this week's talks in Geneva. The North Korean negotiator, Vice Foreign Minister Kim Kye Gwan, cited “big improvements” in some areas and said the remaining differences will be resolved in future meetings.

In Washington, State Department spokeswoman Victoria Nuland said the envoys also discussed North Korea's request for international food aid to cope with shortages attributed to floods and mismanagement.

She said the American response will depend on a U.S. needs assessment and competing demands for famine relief elsewhere, including the Horn of Africa. She said the U.S. also wants assurances that aid reaches North Koreans who are truly in need.

The six-party talks are aimed at getting North Korea to end its nuclear weapons program in return for diplomatic and economic concessions. They involve the United States, North and South Korea, Russia, Japan and China.