The South Korean goverment had told the International Atomic Energy Agency in August that its scientists carried out secret experiments in enriching uranium four years ago, the agency said Thursday.

A group of IAEA inspectors went to Seoul Sunday for a week-long visit after the experiment was reported in accordance with strengthened safeguard regulations.

"We expect that the agency will fully investigate the matter and keep the Board of Governors fully informed," US State Department spokesman Richard Boucher said.

"It is important that all such activity be investigated. When the investigation is complete, we and other members of the board will be able to draw the appropriate conclusions," he said. "So that needs to be done."

He said Washington was in touch with the IAEA and the South Korean government on the issue, which observers felt had embarassed Washington at a time when it was working with Seoul to pressure North Korea to dismantle its nuclear weapons program.

Following demands by the United States, South Korea had officially terminated in the 1970s its efforts to develop nuclear weapons.

Boucher said the information about South Korea's activity was provided to IAEA as part of Seoul's "initial declaration" under an additional protocol to the Nuclear Non-Proliferation Treaty, which the country voluntarily adopted in February.

"I would say that South Korea has voluntarily reported this activity. They are cooperating fully and proactively in order to demonstrate that the activity has been eliminated and it is no longer cause for concern," he said.

"Their transparency and cooperation in resolving this matter is a strong example of how states should respond in complying with their obligations under the Non-Proliferation Treaty."

Asked why the United States was not critical of South Korea, Boucher said: "what they had done in the past was activity that should not have occurred.

"It's activity that should have been reported under the additional protocol."

Boucher also said that the scale of South Korea's enrichment activity was "much, much smaller than that being discussed in the situations of North Korea or Iran."

The South Korean's revelation seems to have complicated IAEA's efforts to get to the bottom of Iran's nuclear program as Teheran could now argue that it should be treated as leniently as Seoul for breaches of IAEA agreements.

The United States and South Korea are involved in six-party talks for about a year with Russia, China, Japan and North Korea to end Pyongyang's nuclear weapons drive.

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