The head of the International Atomic Energy Agency (IAEA) says South Korea's undeclared nuclear experiments are a matter of serious concern, and a detailed investigation is underway.

IAEA director Mohamed ElBaradei told reporters Monday he is concerned that South Korea had only recently informed the agency about experiments involving uranium enrichment that took place four years ago.

"Clearly any activity that involves the separation of plutonium or the enrichment of uranium are matters of serious concern from a proliferation perspective and therefore we are going to treat them with the seriousness they deserve," he said.

Mr. ElBaradei told the IAEA board of governors that inspectors visited three previously undeclared facilities in South Korea. He says they discovered that South Korea had produced about 150 kilograms of natural uranium metal in the 1980s, which was used in experiments nearly twenty years later.

A diplomat familiar with the IAEA, who did not want to be named, said this link raises many questions on the depth and history of a project that possibly spanned two decades.

The head of the South Korean delegation, Cho Chang-bom, told reporters this was an "unfortunate" experiment carried out by ambitious scientists.

"These experiments were done by a small group of scientists for research purposes on a laboratory scale and without the knowledge or authorization of the government of the Republic of Korea," he said. "And the government of the Republic of Korea, as you are all aware, did not have an enrichment or reprocessing program at all, and we do not have at the moment and we will not have that enrichment or reprocessing facilities."

Mr. Cho said the research reactor was subsequently dismantled. The envoy stressed that Seoul is committed to a nuclear-free Korean peninsula and would fully co-operate with the IAEA in its investigations.

The IAEA experts are analyzing photographs, records and environmental samples taken at the sites. Director ElBaradei says the agency wants to interview scientists and view the dismantled equipment before making another report to the board in November.