South Korean government scientists secretly enriched uranium to nearly bomb-grade levels in experiments conducted four years ago, officials in Seoul and Vienna acknowledged yesterday, as the International Atomic Energy Agency announced it had launched a major investigation of the country's programs and nuclear technologies.

U.S. officials, speaking on the condition of anonymity, said the United States had begun a separate inquiry into whether the scientists involved had trained at U.S. nuclear facilities as part of friendly exchange programs and whether the technology may have come from the United States years ago.

Experts and diplomats said revelations that a U.S. ally conducted secret nuclear work, in violation of the Nuclear Nonproliferation Treaty, could complicate efforts by the Bush administration to increase international pressure on Iran and North Korea, which are also accused of conducting clandestine programs.

The administration wants Iran referred to the U.N. Security Council for violating its commitments to the treaty. But South Korea, also a signatory to the treaty, could face the Security Council first, some diplomats said on the condition of anonymity.

State Department spokesman Richard A. Boucher said during a briefing Wednesday that the South Koreans should not have been doing such work, but he applauded their disclosures and cooperation with the IAEA.

The agency dispatched a team to South Korea over the weekend. The inspectors will report back to IAEA headquarters in Vienna early next week, agency spokesman Mark Gwozdecky said in a statement. The findings will be presented to the IAEA board when it meets on Sept. 13.

The treaty requires all members to report any uranium-enrichment activities to the IAEA. Not doing so is considered a serious violation.

Confronted by mounting suspicions within the IAEA, the South Koreans admitted to the activity on Aug. 23 in a written declaration to the agency. But in statements Wednesday, Seoul said that it was a one-time experiment conducted without government authorization and that it was geared toward the country's nuclear energy program.

South Korean officials emphasized that only a tiny amount of uranium was enriched. But diplomats in Vienna said the amount was enriched to nearly bomb-grade level -- a technological feat few countries have achieved.

The officials also said the facility was dismantled after the experiment. Inspectors are trying to determine how long it took the South Koreans to complete the experiment, where they got the uranium and when they discontinued the research.

"Not only did they have an undeclared uranium-enrichment program, but they were actually making something close to bomb-grade, so you have to conclude someone wanted to develop a capability to make nuclear weapons," said David Albright, a former IAEA inspector and head of the Washington-based Institute for Science and International Security.

The South Korean scientists, working at a government laboratory, used laser isotope separation to enrich the uranium. Experts said the laser process, which identifies and then extracts uranium isotopes for enrichment, has no practical civilian applications and is not used in any nuclear energy program.

Countries such as the United States long ago gave up on the possibility of using the process commercially because it was too difficult and expensive.

The announcement came as the Bush administration launched new diplomatic efforts aimed at increasing pressure on Iran for hiding nuclear programs that it also maintains are peaceful.

Secretary of State Colin L. Powell said Wednesday that he will begin calling his counterparts in France, Britain and Germany to strategize on Iran ahead of the IAEA meeting.

"This could not have come at a worse time for the Bush administration's efforts on both Iran and North Korea," said Jon Wolfsthal, a nonproliferation specialist at the Carnegie Endowment for International Peace. "Iran is going to say the U.S. is giving an ally a free pass, while the North Koreans are going to accuse the U.S. and the South of hypocrisy and warmongering."

A U.S. official involved in setting North Korea policy expressed concern that North Korea would use the revelations to its advantage and negatively affect six-party talks among the United States, China, Russia, Japan and South Korea aimed at defusing a crisis over Pyongyang's nuclear program.

After months of tensions between the United States and North Korea, Pyongyang threw out IAEA inspectors in December 2002 and walked away from the treaty shortly afterward, a move that heightened suspicions the country was continuing to develop nuclear capability.

The United States helped persuade South Korea to give up a secret weapons program in the 1970s, and over the years has discouraged it from buying materials that could have nuclear weapons applications.

Seoul's disclosure was made in a written declaration detailing its compliance with the nonproliferation treaty.

Daryl G. Kimball, head of the Arms Control Association, said the South Korean experiment "underscores the value and need for better international verification. This needs to be the universal standard that all states have to live by," he said.

Special correspondent Cho reported from Seoul.

Secretary of State Colin L. Powell said he will consult with officials in other countries on how to deal with Iran's nuclear programs.