US State Department spokesman Richard Boucher said here on Friday that South Korean nuclear tests in the past were "laboratory experiments."

"We don't see these as nuclear weapons activities," Boucher said at a regular news briefing.

He said there's no comparison between South Korea and the Democratic People's Republic of Korea (DPRK) in terms of their nuclear activities.

South Korean nuclear activities, occurred many years ago, were different both in scale and type from that of the DPRK, which, Boucher said, are "to develop sources of enriched uranium for the purpose of nuclear weapons."

Seoul last week admitted South Korean scientists had carried out an unauthorized experiment to enrich uranium four years ago but said it was not linked to any weapons program.

South Korea has had the experiment reported to the UN nuclear watchdog the International Atomic Energy Agency.

Following its acknowledgment of its enrichment of uranium, South Korea said Thursday that it extracted a tiny amount of plutonium in a nuclear experiment in 1982.

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

The DPRK has warned that South Korea's uranium enrichment experiment could fuel an arms race in northeast Asia.

Source: Xinhua