The Stalinist North refused to attend a fourth round of talks proposed for last month, citing what it called "hostile" US policy and South Korea's revealations of past undeclared nuclear experiments.

The International Atomic Energy Agency (IAEA) sent inspectors to South Korea twice last month after Seoul revealed that its scientists secretly enriched a tiny amount of plutonium in 1982 and uranium in 2000.

"The DPRK (North Korea)...holds that this issue should be clarified before anything else at the multilateral negotiations for the denuclearization of the peninsula," the North's official Korean Central News Agency said.

The agency said the IAEA and its chief Mohamed ElBaradei are biased by going easy on Seoul in nuclear issues.

Visiting Seoul earlier this month, ElBaradei praised South Korea for cooperating fully with the IAEA inspections. He criticised the UN Security Council for failing to take action against the North over its nuclear weapons program.

South Korea says its laboratory experiments were scientific research not linked to any weapons programs. It allowed full-scale IAEA inspections which are still under way.

Seoul opposes bringing its own nuclear issue to the six-way talks which also involve the United States, Japan, China and Russia.

The nuclear stand-off began in October 2002 when the United States accused North Korea of operating a nuclear weapons programme based on enriched uranium in violation of a 1994 agreement.

Pyongyang has denied running the uranium-based program but has restarted its plutonium program.

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