South Korea acknowledged Thursday that it conducted a plutonium-based nuclear experiment more than 20 years ago, shortly after it admitted to scientific tests involving uranium.

Plutonium (search) and enriched uranium (search) are two key ingredients of nuclear weapons. The controversy over South Korea's uranium-based experiment has threatened to further disrupt troubled efforts to persuade North Korea to dismantle its suspected nuclear weapons programs.

Kim Sung-chul, deputy spokesman of the South Korean Foreign Ministry, said the plutonium experiment was carried out in the early 1980s at a reactor since dismantled at the Korea Atomic Energy Research Institute (search).

The national news agency Yonhap said the experiment conformed to a safeguard agreement that it signed with the U.N. nuclear watchdog.

The revelation follows a disclosure last week that the U.S. ally conducted a secret uranium-enrichment experiment four years ago. North Korea responded to that experiment by warning of a "nuclear arms race" in Northeast Asia.

Kim said the Vienna-based International Atomic Energy Agency has inquired over the years about the plutonium-based experiment, which he described as research activity.

"The experiment in question took place in the TRIGA research reactor in Gongryong in Seoul in the early 1980s," Kim cited a senior ministry official as telling reporters at a briefing. "The reactor has since been dismantled."

North Korea's envoy to the United Nations, Han Sung Ryol, told Yonhap that the communist state found the United States "worthless" as a dialogue partner because it was applying "double standards" to the two Koreas.

"We see South Korea's uranium enrichment experiment in the context of an arms race in Northeast Asia," Han was quoted as saying. "Because of the South Korean experiment, it has become difficult to control the acceleration of a nuclear arms race."

Han's comments were North Korea's first reaction to the South Korean admission this week that its scientists produced a small amount of enriched uranium in 2000.

South Korea said it should have reported the uranium enrichment experiment to the U.N. nuclear watchdog agency. It has denied any intentions of developing nuclear weapons.