The North's five negotiating partners, which includes Japan, have promised Pyongyang one million tons of fuel oil or equivalent aid in return for the disabling of its nuclear plants and a declaration of its atomic programmes.

"Now is the time that Japan should start participating in economic and energy assistance so it can be completed. This is very important," said Seoul's nuclear envoy Kim Sook.

Japan refuses to contribute its share pledged in a landmark 2007 deal until the North fully accounts for Japanese abducted by the communist state during the Cold War.

Kim will fly to Tokyo later Wednesday for talks with his US and Japanese counterparts about preparations for the next round of six-party negotiations. US envoy Christopher Hill will also visit talks host China.

The talks group the two Koreas, the United States, China, Japan and Russia.

"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," Kim said.

Though the Japanese aid was not a precondition for progress in the talks, it would have an impact on them, he said.

The envoy said the six parties "now share a common view on the necessity to hold six-party talks ... and move on to the third phase (of a denuclearisation pact)."

Disabling of the plutonium-producing plants was going ahead but the North is almost six months late in handing over its promised declaration.

Kim said he expects the document to be delivered this month. A new round of six-party talks would then discuss ways to verify it and would prepare for the third and final phase of disarmament.

Under this, the North would dismantle the plants and hand over all nuclear material.

In return, Washington would lift sanctions and establish diplomatic relations, while the parties would negotiate a pact formally ending the 1950-1953 war.

The six-party talks have not been held since last September but there have been several signs of progress in recent weeks.

North Korea has handed over more than 18,000 pages of operating records from its Yongbyon nuclear complex to Washington.

Last week, Japan dropped some of its sanctions against the North after the communist state agreed to reopen a probe into the abductions.

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