The UN nuclear watchdog has said South Korean scientists illegally conducted secret nuclear tests on a larger scale than Seoul had previously declared.
Seoul says the tests were only small-scale
A report by the International Atomic Energy Agency said scientists enriched a small amount of uranium in 2000 to a level almost useable in nuclear arms.
IAEA boss Mohammad ElBaradei said uranium and plutonium tests by South Korea were a matter of serious concern.
Seoul has repeatedly stressed it has no intention of building nuclear weapons.
News of the secret experiments caused extreme embarrassment to South Korea and its main ally, the US.
It has also given North Korea a chance to deflect criticism of its own nuclear activities, and calls for it to rejoin six-nation talks on the issue.
South Korea has admitted that its scientists conducted, without official authorisation, tests in 1982 to extract plutonium and in 2000 to enrich uranium - two separate routes to an atomic bomb.
But the government has argued that the tests were on too small a scale to be significant and only 0.7g of plutonium and 200mg of uranium were produced.
However, South Korea's concealment of its secret tests is seen by some experts as violation of Seoul's obligations under the nuclear Non-Proliferation Treaty (NPT).
They say it could require the IAEA to refer South Korea to the UN Security Council.
The IAEA report - which the BBC has obtained - said that South Korea had failed to inform the agency about the secret experiments, the BBC's Bethany Bell in Vienna reports.
"Although the quantities of nuclear material involved have not been significant, the nature of the activities - uranium
enrichment and plutonium separation - and the failures by [South Korea] to report these activities in a timely manner... is a matter of serious concern," said the report.
"The agency is continuing the process of verifying the correctness and completeness of [Seoul's] declarations," it added.
The report said that South Korean scientists had enriched a small amount of uranium to 77% uranium-235, which is close to weapons-grade.
However, it said the average enrichment during the uranium experiments was about 10.2%.
The report stated that the IAEA had found no indications that the experiments had gone beyond small-scale laboratory activities.
Mr ElBaradei also praised Seoul's co-operation with the investigation into the matter.