Washington says South Korea may have been involved in illicit nuclear activities even before its recently disclosed experiment to enrich uranium.
A senior U.S. official, speaking on condition of anonymity, said Wednesday that it is Washington's "understanding that over 20 years ago, Seoul performed experiments involving trace amounts of plutonium."
Highly enriched uranium and plutonium are the two main ingredients of nuclear bombs.
The U.S. revelation follows Seoul's disclosure last week that it conducted a secret uranium enrichment experiment four years ago but that it was not linked to any weapons program.
An International Atomic Energy Agency (IAEA) team was sent to South Korea last week to carry out a related probe. Its findings will be discussed at a four-day board meeting of the nuclear watchdog starting Monday.
The U.S. official's comments come after North Korea warned of a "nuclear arms race" in Northeast Asia in response to Seoul's acknowledgement of the uranium-enrichment experiment.
North Korea's envoy to the U.N., Han Sung-ryol, told Seoul-based Yonhap News Agency on Wednesday that his country found the United States "worthless" as a dialogue partner because it was applying "double standards" to the two Koreas.
Han called South Korea's uranium enrichment experiment "a dangerous move that could accelerate a nuclear arms race in Northeast Asia."