ElBaradei told a meeting of the International Atomic Energy Agency's board of governors that he would report again to the board by its next meeting in November.

"It is a matter of serious concern that the conversion and enrichment of uranium and the separation of plutonium were not reported to the agency" as required under the nuclear Non-Proliferation Treaty (NPT), ElBaradei said, according to a copy of his speech made available to reporters.

South Korea's ambassador to the IAEA Cho Changbeon told reporters that there would be no more revelations of illicit nuclear activities.

"I think I can safely say that, according to the investigations the South Korean government has conducted, there are no other such, similar experiments on enrichment or processing," Cho Changbeon said.

He said the nuclear experients in 2000 and in the 1980's were done by overambitious scientists and not authorized by the government.

South Korea does "not have enrichment facilities" or a weapons program and will continue to honor its NPT obligations, he said.

The United States said Friday that South Korea's nuclear research was not linked to any manufacture of atomic weapons.

State Department spokesman Richard Boucher described the South Korean research involving both plutonium and enriched uranium as "laboratory experiments" and said they were "certainly a different scale and type" from "North Korea's efforts to develop sources of enriched uranium for the purpose of nuclear weapons."

South Korea, a close ally of the United States, has admitted that its scientists had extracted a small amount of plutonium, a key ingredient for making nuclear bombs, in secret research in the early 1980s.

The admission came after Seoul said its scientists had conducted unauthorised experiments to enrich uranium, which is also used to build atomic weapons.

The revelation embarassed both the United States and South Korea which are trying to pressure North Korea to end its nuclear weapons drive.

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