SEOUL, Oct 2: North Korea on Saturday ruled out any progress in inter-Korean relations unless the South's secret nuclear experiments are thoroughly investigated.

Relations between the two Koreas have been strained over Seoul's shock revelations that South Korean scientists carried out clandestine nuclear experiments and a mass defection by North Korean refugees to the South in July.

"It will be impossible to expect any development of the inter-Korean relations unless the truth about South Korea's secret nuclear experiments is probed," the North's official Korean Central News Agency quoted a spokesman of the Committee for the Peaceful Reunification of the Fatherland saying in a statement.

The committee is Pyongyang's counterpart of the South's Unification Ministry which handles official contacts with the North.

Last month, South Korea admitted that its scientists extracted a small amount of plutonium - a key ingredient for making nuclear bombs - in secret research in the early 1980s and had also conducted unauthorised experiments to enrich uranium, also used to build nuclear weapons.

Seoul has said the experiments were not linked to an atomic weapons programme but the clandestine activity has embarrassed the United States and ally South Korea at a time when they are trying, through six-party talks, to pressure Stalinist North Korea to end its nuclear weapons drive.

Pyongyang however said Seoul's nuclear revelations were only "the tip of the iceberg". "The cases of secret nuclear experiments disclosed one after another in South Korea recently are all but a tip of the iceberg of the serious nuclear development activities that the South Korean authorities have so far conducted in a premeditated manner," the North Korean statement said.

"The clandestine nuclear development pursued for over 20 years helped South Korea acquire basic criteria for nuclear armament such as extraction of nuclear substance, production of nukes, access to means for nuclear delivery and preparations for a nuclear war and put all this under a perfect system.

"This is a serious case as it seriously affects the situation on the Korean peninsula and the relations between the North and the South."

Pyongyang has also refused to take part in a fourth round of multilateral talks on its nuclear weapons programme which had been scheduled for September, blaming both US "hostile" policy and secret nuclear experiments in South Korea.

The nuclear stand-off intensified 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.-AFP