But the International Atomic Energy Agency (IAEA) said the tests were experimental and small-scale, and that South Korea had cooperated with the agency in investigating the matter, in a report released Thursday.

The IAEA report could clear the way to the agency bringing South Korea before the UN Security Council for the lapses when the IAEA's board of governors meets in Vienna on November 25.

This could prove embarrassing to South Korea's main ally the United States which is leading a drive to get North Korea to give up plans to make nuclear weapons.

Washington is also calling on the IAEA to report Iran to the Security Council for similar breaches of safeguards monitoring authorized by the nuclear Non-Proliferation Treaty (NPT) that it says are in Tehran's case proof of a hidden nuclear weapons program.

But the report says the IAEA investigation into South Korea is continuing and this could lead IAEA board members to put off ruling on the South Korean dossier.

South Korea had in August admitted to the IAEA that its scientists had conducted secret experiments in separating plutonium in the 1980's.

Seoul also reported laser enrichment of uranium "in 2000 by scientists at the Korea Atomic Energy Research Institute (KAERI) in Daejeon" that refined uranium to an average level of 10.2 percent and up to the highly enriched level 77 percent, which is close to weapons-grade, according to the report.

The IAEA made the uranium enrichment public in September, even if Seoul at first denied producing highly enriched uranium.

The IAEA report said it has now discovered that South Korean scientists had also made natural uranium metal which is used in enriching uranium and in 1979-1981 conducted some experiments in enriching uranium chemically.

South Korea claims these were all small laboratory experiments that produced only 200 milligrams of enriched uranium, that they were done without government approval and that such activity has been stopped.

The IAEA report said South Korea produced 0.7 grams of plutonium in 1982, but added that this had an "isotopic content of about 98 percent of PU-239," which is weapons grade.

These amounts are very small as it takes from 15-25 kilograms of highly enriched uranium to make an atomic bomb while the amount for plutonium is seven kilograms.

The IAEA said that "although the quantities involved have not been significant, the nature of the activities ... and the failures by the ROK to report the activities in a timely manner in accordance under its Safeguards Agreement is ... a matter of serious concern."

"They haven't told us things they should have" but since then "they have provided a lot of good cooperation," a diplomat close to the IAEA said.

The report said South Korea has provided "active cooperation," including access to sites and allowing environmental swipe sampling.

"There is no indication that these undeclared experiments have continued," the diplomat said.

The report said "at least ten" laser enrichment related experiments "were carried out at KAERI facilities between 1993 and 2000" and that South Korea has said "these experiments were authorized only by the president of KAERI in Daejeon, involved some 14 KAERI scientists and were conducted in the broader context of a stable isotope separation project."

North Korea, citing concern about Seoul's secret nuclear experiments and "hostile" US policy, boycotted in October a planned new round of six-nation talks aimed at ending the communist state's atomic weapons drive.

But US Secretary of State Colin Powell said in October that there was no comparison with secret nuclear experiments carried out by South Korea in the past and ongoing atomic programmes in North Korea and Iran.

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