The South Korean government acknowledged Thursday that it extracted a small amount of plutonium during a 1982 research experiment, a declaration that came a week after the country acknowledged its scientists had secretly enriched uranium.

Diplomats at the International Atomic Energy Agency in Vienna said the agency had begun to suspect that South Korea was conducting nuclear experiments more than six years ago and said South Korean officials had worked hard to hide the experiments from inspectors.

"They had a fairly elaborate plan involving denial and deception in order to evade detection by inspectors," said one diplomat who would discuss the agency's investigation only on condition of anonymity.

South Korean Foreign Ministry officials called those accusations "groundless and unsubstantiated" and said they had fully cooperated with inspectors and would continue to do so.

In Washington, U.S. officials said they gave a clear message to South Korea this week that they consider the charges to be serious and would apply the same standards to any country found to be violating the nuclear Non-Proliferation Treaty.

That message, which diplomats said would be repeated next week in Vienna at a board meeting of the IAEA, was meant to assuage concerns that the United States was applying a double standard by pushing for tough action against North Korea and Iran, which have also been accused of conducting clandestine nuclear work.

The IAEA believes that South Korea's work on plutonium and uranium -- the key ingredients for nuclear weapons -- seriously violated the treaty and that the matter could be referred to the U.N. Security Council in November, diplomats said.

One diplomat familiar with the IAEA's work said that despite South Korea's official denials, uranium was secretly enriched in 2000 to nearly bomb-grade levels and the other experiment was optimized to produce bomb-grade plutonium. On Friday, South Korean officials again disputed that their experiments had reached anywhere near bomb-grade levels.

South Korea, which derives 40 percent of its energy from nuclear power, contends that all the tests were one-time research efforts unrelated to weapons programs.

The IAEA announced last week that it had launched an intensive investigation after South Korea belatedly admitted to enriching a small amount of uranium during three experiments in January and February of 2000 -- tests that diplomats and experts said the South Korean government was required to report under terms of the Non-Proliferation Treaty.

North Korea, which has been pressured by the United States about its nuclear program, reacted quickly to the report on South Korea. On Wednesday, North Korea's ambassador to the United Nations, Han Sung Ryol, said the Bush administration had a "double standard" on the Korean Peninsula and warned of a budding "nuclear arms race" in northeast Asia.

North Korea expelled international inspectors and withdrew from the Non-Proliferation Treaty about two years ago, and U.S. intelligence officials have said they believe the North Koreans have now amassed an arsenal of up to eight nuclear devices. After three rounds of six-nation talks on the North Korean nuclear program in Beijing, the Pyongyang government and the Bush administration have not significantly changed their negotiating positions. Analysts are concerned about progress in the talks, predicting they may be delayed until after the U.S. presidential election in November.

"This gives another reason for North Korea to raise the issue of fairness with the international community," said Jhe Sung Ho, professor of law at Joongang University in Seoul. "They are going to claim that Washington is pressing them while giving South Korea a break."

South Korea conducted nuclear weapons research during the 1970s but is believed to have abandoned it under U.S. pressure before the end of the decade. One South Korean official familiar with the government's report to the IAEA on the 1982 plutonium experiment said details of the test remained sketchy but insisted there was no indication it had been related to a weapons program.

"This experiment was conducted by a small group of scientists to analyze the chemical characteristics of plutonium," the Science and Technology Ministry said in a statement. "We have no written data left on the result of the experiment and the amount of plutonium extracted, but we estimate that a very minute amount in the range of milligrams was extracted." But one South Korean official familiar with the findings said if the experiment had taken place today, "the government would not have allowed it."

The first indication of a plutonium experiment came to light in 1998 after international inspectors detected traces of the substance at a government-run nuclear research center in Seoul, according to the South Korean science ministry. IAEA sources said the samples were inconclusive, and inspectors began additional testing in other areas of the country. The South Korean government said the IAEA made only a "casual inquiry" by fax in 1998 and submitted an official request about the incident in 2003.

During that work, the South Koreans allegedly dismantled a test site, moved equipment and failed to notify the IAEA about the experiments while they knew the agency was trying to determine whether such tests had been conducted, according to the diplomats. By 2003, inspectors had collected irrefutable evidence of plutonium reprocessing and uranium enrichment, and they confronted the South Koreans with it last December.

The Seoul government submitted a report on the plutonium incident this March, but the report faced delays and problems, officials said, because the key researcher on the project had died. An official familiar with the case would not identify the researcher and could not cite the date of his death.

The IAEA has identified six violations by the South Koreans that could be reported to the Security Council.

The plutonium experiment took place during political turmoil in South Korea following the 1980 military coup by former president Chun Doo Hwan, who left office with the return to democracy in 1987. The South Koreans said they were unsure if the IAEA would declare the plutonium test in violation of international laws. They disclosed information about the plutonium experiment after the Associated Press quoted an unnamed senior Bush administration official in Washington, who gave details.

"We haven't found out the accurate purpose of the experiment, because the head of the research project at that time has passed away," said one South Korean official familiar with the plutonium test.

But Shin Sung Tack, a nuclear expert at the government-run Korean Institute for Defense Analysis, said, "You need at least 10 kilograms of plutonium to make low- level weapons grade." That is far beyond what the South Koreans said their scientists produced. High-ranking South Korean officials insisted they did not know about the uranium enrichment experiments until lower-level government administrators informed them in February.

Linzer reported from Washington. Special correspondent Johee Cho contributed to this report.

Workers dismantle the facilities of a research reactor in Seoul. The IAEA said South Korea tried to hide nuclear tests from international inspectors.