In a startling revelation, the South Korean government told the International Atomic Energy Agency that its scientists had carried out uranium enriching experiments four years ago without Seoul's knowledge and approval.

Analysts said the revelation, announced by the IAEA on Thursday, could hinder current multilateral talks aimed at pressuring Stalinist North Korea to end its nuclear weapons drive and spark a nuclear arms race in Northeast Asia particularly.

Some even called for South Korea to be hauled up before the UN Security Council, just like its northern neighbor, for any attempts to stifle plans to have a nuclear-free Korean peninsula.

IAEA inspectors went to the South Korean capital Sunday for a week-long visit after the experiment was reported in accordance with strengthened safeguard regulations under a new protocol to the Nuclear Non-Proliferation Treaty, which the country voluntarily adopted in February.

"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. The Vienna-based IAEA board meets September 13.

"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 under the six-party talks 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 highlighted that South Korea had voluntarily reported the unapproved activity and was "cooperating fully and proactively in order to demonstrate that the activity has been eliminated and it is no longer cause for concern."

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."

South Korea's revelation has 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, analysts say.

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.

Jon Wolfsthal, deputy director of non-proliferation during the Clinton administration, said the South Korean disclosure would give North Korea "a field day on the propaganda front" and prolong resolution to the Korean nuclear crisis.

"I think the United States should take South Korea to the UN Security Council" to set a "good example" for countries like Iran, which is suspected to be harboring nuclear weapons, he said.

He said if North Korea's nuclear program continued unconstrained, not only South Korea but Japan and Taiwan could consider ways to secure their security through a nuclear option.

"These are countries that have the technical capability to build nuclear weapons but have made a political decision not to do so," he said.

China is the only bona fide nuclear state in Northeast Asia although North Korea, which opted out of the Nuclear Non-Proliferation Treaty in January 2003, is strongly believed to have nuclear weapons capability.

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