The experiment conducted in January 2000 at the country's state-run nuclear research center produced a minuscule amount of enriched uranium.

Until now, the government has argued that it saw no wrongdoing despite its failure to report the experiment that produced 0.2 grams (0.007 ounces) of uranium to the nuclear watchdog, International Atomic Energy Agency (IAEA).

A senior foreign ministry official indicated at a press briefing that the government was now stepping back from that position.

"This experiment itself was not subject to declarations to the IAEA but we should have declared the nuclear materials," said the official who declined to be named. "But the government failed to recognize and declare it."

IAEA staff inspected the research center in South Korea last week where the experiment took place and returned to Vienna with a 0.1-gram sample of the enriched uranium.

South Korea will send a team of officials to Vienna to attend a four-day board meeting of the IAEA starting Monday that will consider the case.

The team will argue vigorously that the "one-off" experiment was conducted for purely academic purposes and was in no way linked to nuclear weapons ambitions.

The South Koreans say the average enrichment achieved during the 2000 experiment was 10 percent, nowhere near the 90 percent needed for nuclear weapons, while the amount separated was microscopic compared to the five kilograms of enriched uranium needed for a bomb.

"The government has maintained transparency and reliability in its non-proliferation policy by voluntarily declaring it and fully cooperating with the IAEA inspection," South Korean Foreign Minister Ban Ki-Moon said in a weekly briefing.

Ban said he expected "the IAEA to handle the case in a factually-correct and balanced manner" at its board meeting.

Revelations that scientists in South Korea engaged in clandestine uranium enrichment embarrassed officials here at a time when Seoul is playing a leading role in efforts to end North Korea's nuclear weapons programme.

However, the government until now has rejected charges it violated its international obligations, claiming there was no stipulation at the time under the nuclear Non Proliferation Treaty to report the enrichment activities.

Seoul argued it was only obliged to report the enrichment activities after new, tougher safeguards came into force under an additional protocol to the treaty that took effect in February this year.

In any case, the government said, it was not told about the experiment at the time and was only informed in June by scientists at the Korea Atomic Energy Research Institute in Daejeon, 160 kilometers (100 miles) south of the capital.

The experiment using laser isotope separation technology was a "one-off" case and South Korea has no interest in running a nuclear weapons programme, the government maintained.

South Korea has the world's sixth-largest civilian nuclear industry and runs 19 nuclear power plants that produce 40 percent of the country's energy needs.

Seoul imports 370 million dollars worth of enriched uranium each year to fuel the power plants.

But South Korea maintains a policy of neither enriching nor reprocessing nuclear fuels on its own territory, a top foreign ministry official said.

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