"It's important that all countries comply with their safeguard obligations," President George W. Bush's spokesman Scott McClellan told reporters.

He noted that the South Korean government had agreed to fully cooperate with investigations into the issue by the International Atomic Energy Agency (IAEA), the UN global nuclear watchdog, and prevent any further recurrence of such activity.

In a startling revelation, the South Korean government informed the IAEA that its scientists had carried out uranium enrichment experiments four years ago without its knowledge and approval, the agency said Thursday.

IAEA inspectors arrived in South Korea Sunday to probe reports that uranium had been enriched at a government research laboratory, South Korean and IAEA officials said.

"The South Koreans disclosed this and said it was a rogue scientist," McClellan said.

His statement appeared to contradict that of the top official of the South Korean Foreign Ministry dealing with the IAEA.

The official, Oh Joon, had insisted that the experiment using laser isotope separation technology was a "one-off" case and the researchers involved were not "rogue" scientists.

South Korea's science and technology ministry said that a "small amount of 0.2 grams (0.007 ounces) of uranium" had been produced during an experiment in January and February 2000.

Washington is in touch with IAEA and the South Korean government on the issue.

The revelation has embarrassed the United States and South Korea at a time when they were involved in six-party talks to persuade North Korea to dismantle its nuclear weapons program.

When asked how this would affect the multilateral talks, McClellan said, "We will continue to pursue a diplomatic approach through the multilateral talks.

"All parties are sending a message to North Korea about the need for North Korea to abandon its nuclear ambitions," he said.

Aside from the United States and the two Koreas, the other parties at the talks are Russia, China and Japan.

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