The North's partners -- the United States, China, South Korea, Russia and Japan -- agreed to share the aid evenly, but Japan refuses to contribute until the North accounts fully for Japanese kidnapped by the communist state during the Cold War to train its spies.

US State Department spokesman Sean McCormack appeared to indicate there was time to resolve the problem when he said Russia was next in line to deliver the fuel oil.

"First things first, Russia's next up," McCormack told a reporter who asked if the supplies would come from North Korea's existing partners or from outside, as some reports suggest.

"I think there's a high degree of confidence among the five that we will meet our obligations," McCormack added during the daily press briefing in Washington.

In Seoul, a senior South Korean official said the six-country forum negotiating North Korea's nuclear disarmament may seek an outside donor if Japan persists in withholding its share of energy aid.

The official, speaking on the condition of anonymity, said South Korea has been discussing the issue with Japan, the United States, China and Russia to keep the aid-for-disarmament accord with the North moving forward.

Under the February 2007 deal, North Korea was to receive one million tons of fuel oil or equivalent energy aid from the other five countries in return for disabling its plutonium-producing plants.

Media reports have said that Australia and the European Union may be approached as alternative suppliers. But neither the South Korean official nor McCormack commented on the reports.

Japan's Yomiuri Shimbun newspaper said the US is preparing to ask several countries, including Australia, to take over Japan's part of the energy assistance.

Japan is expected to accept the US proposal during the next six-way talks expected in November, the paper's evening edition reported.

North Korea has repeatedly demanded that Japan quit the six-party talks.