South Korea (search) more than 20 years ago secretly conducted an experiment with traces of plutonium (search), a key ingredient in making nuclear weapons, a senior Bush administration official disclosed Wednesday.

The revelation follows disclosure last week that the U.S. ally had conducted four secret uranium-enrichment experiments four years ago.

North Korea (search) responded on Wednesday to the uranium-enrichment experiments by warning of a "nuclear arms race" in Northeast Asia.

The United States, with the support of South Korea, Japan, China and Russia, has been trying to negotiate an end to North Korea's nuclear weapons program. The talks are due to resume at the end of the month, but no date has been announced.

Plutonium and enriched-uranium are the two key ingredients of nuclear weapons.

South Korea is discussing its actions with the U.N. International Atomic Energy Agency in Vienna, said the senior official, who spoke on condition of anonymity.

The administration is aware generally of the content of South Korea's reporting to the IAEA on nuclear experimental activity conducted in past years, another U.S. official said. The administration is confident the agency will thoroughly pursue any inconsistencies or questions, said the official, speaking on condition of anonymity.

Meanwhile, North Korea's envoy to the United Nations, Han Sung Ryol, told the South Korean national news agency Yonhap that it found the United States "worthless" as a dialogue partner because it applied double standards to the two Koreas. This could be a tip-off that North Korea will resist or delay efforts to halt its weapons program.

The State Department last week criticized South Korea for its secret work on uranium-enrichment while praising South Korea for working with the IAEA to make sure the program had ended.

The uranium and plutonium disclosures came amid a strenuous effort by the Bush administration to stop Iran from beginning a uranium-enrichment program that U.S. officials say could produce four nuclear weapons.

"We are in touch with the South Korean government," State Department spokesman Richard Boucher said Wednesday when asked whether South Korea had experimented with plutonium.

"We are also aware generally of what the South Koreans reported to the International Atomic Energy Agency on nuclear experiments conducted in past years," he said.

"We have confidence that the agency will pursue all these matters," said Boucher, adding that he would withhold comment "as far as other possibilities" apart from the uranium-enrichment experiments.

South Korea is in the process of verifying to the U.N. agency that its uranium-enrichment activity "has been eliminated and will not be repeated," Boucher said last week.

"But what they had done in the past was activity that should not have occurred," he said. "It's activity that must be eliminated, and we are glad that South Korea is working in a transparent manner to do that."

The spokesman said the scale of South Korea's enrichment work was much smaller than that of North Korea and Iran. And he called on North Korea to disclose its activity to the U.N. agency.

Daryl G. Kimball, executive director of the Arms Control Association, said even after South Korea ratified the Nuclear Non-Proliferation treaty in 1975, pledging not to try to develop nuclear weapons, Seoul was suspected of trying to keep nuclear options open.

It had a nuclear weapons research program in the early 1970s but the conventional wisdom was that South Korea did not acquire technology for enriched uranium or plutonium, Kimball said.

The disclosure Wednesday indicates efforts to close down the program were not completely successful, the head of the private research group said in a telephone interview.

"The devil is in the detail here, but in terms of newness I think that this probably is a surprise but it is not particularly shocking, either," he said.

Still, Kimball contrasted what South Korea had done as not an immediate problem compared to North Korea's current, active pursuit of nuclear weapons.