"Our understanding is that over 20 years ago, the South Koreans did experiments involving trace amounts of plutonium," a senior US official said, speaking on condition of anonymity.

Highly enriched uranium and plutonium could be used to make nuclear bombs.

Seoul last week admitted that South Korean scientists had carried out an unauthorised experiment at its state-run nuclear research center to enrich uranium four years ago but says it was not linked to any weapons program.

The clandestine activity embarrassed both the United States and its ally South Korea at a time when they are trying to pressure North Korea to end its nuclear weapons drive through six-party talks hosted by China.

The International Atomic Energy Agency (IAEA) team went to South Korea to carry out a probe last week and its findings will be discussed at a four-day board meeting of the nuclear watchdog starting Monday.

Asked whether the United States had been talking to South Korean on any activity beyond the uranium enrichment experiment, specifically involving plutonium, US State Department spokesman Richard Boucher said: "I think I'd just have to say in general terms we are in touch with the South Korean government."

He said Washington was aware of what the South Koreans had reported to the IAEA on nuclear experiments conducted in past years.

"We have confidence that the agency will pursue all these matters, including any questions that might arise from the declaration," Boucher told reporters.

"So at this point, as far as other possibilities and things, I withhold comment," he said.

On whether South Korea had indulged in illicit nuclear activity other than what it had already admitted, Boucher said: "Our understanding is that all these activities were in the past and some of them quite a while back in the past."

During the military dictatorship of Park Chung-hee in the 1970s, Seoul launched a secret nuclear weapons program, but the United States reportedly persuaded it to abandon the plan.

Boucher reiterated that the United States was satisfied South Korea was reporting its nuclear activities to the IAEA.

But North Korea has stepped up criticism of its southern neighbor and accused the United States of practising "double standards."

"We view South Korea's uranium enrichment program as part of armament race in the Northeast Asian region," Han Song-ryol, deputy chief of North Korea's mission to the United Nations, told the Yonhap News Agency.

"It will be difficult to prevent the spread of armament race in the region due to the South's nuclear experiment.

Han said Pyongyang would take issue with the matter, casting doubt on multilateral talks aimed at resolving the impasse over North Korea's nuclear weapons program, Yonhap said.

The fourth round of six-nation talks, involving China, the two Koreas, Japan, Russia and the United States, aimed at persuading Pyongyang to give up its nuclear ambitions, are expected in Beijing this month.

Global environmental group Greenpeace has said that South Korea's illicit nuclear activity revelation was a "stark warning" about the nuclear threat on the Korean peninsula and the wider Northeast Asian region.

Japan currently has a plutonium stockpile of some five tonnes of plutonium, it says. North Korea is believed to have already acquired nuclear weapons.

China is the region's only "official nuclear power."

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