Seoul  - South Korea called Wednesday on Japan to immediately start energy shipments to North Korea to spur on negotiations with Pyongyang on ending its nuclear programmes.

Seoul's top nuclear envoy, Kim Sook, said he worried time was running out on the talks, the last round of which was held in September.

"I'm concerned that hesitation at this stage will result in the closure of the window of opportunity that we have worked so hard to open," he said in Seoul before departing for talks in Tokyo with his Japanese and US counterparts.

Japan was part of a six-nation agreement in which it, the United States, China, South Korea and Russia agreed to provide energy and economic aid to the North in return for Pyongyang dismantling its main nuclear facility.

But Tokyo has baulked at providing the aid because of abductions of its citizens by North Korean agents in the 1970s and '80s, which it said remained unresolved.

Last week, however, Japan announced that North Korea had agreed to reinvestigate the cases, and in return, Tokyo said it would partially lift sanctions it imposed after Pyongyang's 2006 nuclear test.

Japan said 17 of its citizens were kidnapped, but North Korea recognized a lower number and had said previously that all had either been returned to Japan or had died.

In its announcement Friday of North Korea's promise to reinvestigate the abductions, Japan called the move a "small step" forward but not enough to take part in energy aid to North Korea.

Pyongyang recently warned of consequences for the six-nation nuclear talks if the aid is not speeded up. (dpa)