"We don't see these as nuclear weapons activities," State Department spokesman Richard Boucher told reporters.

He described the South Korean research involving both plutonium and enriched uranium as "laboratory experiments" and said they were "certainly a different scale and type within 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, admitted on Thursday 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 just a week 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 were trying to pressure North Korea to end its nuclear weapons drive.

The International Atomic Energy Agency (IAEA) board will discuss the issue at a meeting in Vienna next week and compile a written report possibly by the end of November.

Boucher said it was premature to speculate on whether South Korea would be referred to the UN Security Council for any flouting of international regulations.

"We'll have to see what the investigation produces and what outcome it is," he said.

Boucher said there was an obligation on the part of the IAEA board to report any noncompliance to the Security Council, citing past cases involving Iraq, North Korea and, most recently, Libya. "And we'll just have to see."

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