As the Bush administration tries to ratchet up pressure on Iran, emerging details of clandestine nuclear work in South Korea indicate that the U.S. ally was more successful than Tehran in producing the key ingredient for a bomb and used deception to conceal the illegal activity from U.N. inspectors for years.

In interviews late last week, diplomats with knowledge of both covert programs disclosed that South Korean scientists enriched uranium to levels four times higher than did their counterparts in Iran. Seoul conducted those experiments, in violation of the nuclear nonproliferation treaty, two years before Iran did and kept them secret for nearly two years after Iran's came to light, said the diplomats, who would discuss the investigation by the International Atomic Energy Agency only on the condition of anonymity.

The South Koreans appear to have experimented with smaller quantities of uranium than Iran did, and there is no indication that Seoul invested the kind of money and resources that Tehran has put into its program, the diplomats said.

IAEA inspectors have yet to uncover the full scope of the activities of either Iran or South Korea. Until two weeks ago, there were no public indications that South Korea had conducted any weapons-related work, and it was not understood how similar the program was to Iran's efforts.

The South Korean revelations have thrown the Bush administration's efforts on Iran and North Korea into turmoil. Over the weekend, U.S. officials said they were forced to scale back plans to refer the Iran issue to the U.N. Security Council by month's end. And a statement from North Korea on Seoul's nuclear work cast further doubt on U.S. hopes of resuming talks later this month aimed at persuading North Korea to dismantle its nuclear weapons program.

U.S. officials had hoped to push its Iran agenda at the IAEA's board meeting that will begin in Vienna tomorrow. But with little support inside the Security Council for muscling Iran, U.S. officials are backing a competing plan from the Europeans that would give Tehran until late November to suspend suspect nuclear work or face the possibility of council action then.

"We tried, but we had to give up on our 'noncompliance' resolution right now," said one U.S. official. "We're hoping that triggering language, calling on Iran to take a series of steps by a certain deadline or face an automatic referral, will do the trick."

Under the new resolution, the IAEA's board would reconvene at the end of November and judge Iran's compliance based on the full history of its program.

European diplomats said a final draft of the resolution is being worked out and emphasized that the new wording offers no guarantee that the matter would wind up before the Security Council.

U.S. officials have said it is too early to know whether the South Korean issue should be referred to the council, but they worked hard to avoid the appearance of being softer on friends than on foes.

"One thing I can assure is that we will not allow a double standard in terms of how we treat the violations," said John R. Bolton, undersecretary of state for arms control, who negotiated with the Europeans on an Iran resolution in Geneva last week.

The IAEA, which has suspected South Korea of violating the nonproliferation treaty for six years, confronted the Seoul government last December. Several months later, diplomats said, South Korea began to acknowledge the work. Publicly, officials in Seoul said the experiments were one-time efforts by scientists working on their own.

But diplomats challenged those assertions and revealed over the weekend that the Seoul government officially and repeatedly blocked IAEA inspections months after the experiments in 2000 and told the IAEA false cover stories.

"In 2001, the IAEA asked to conduct a regular inspection and was denied. That happened at least twice before the South Koreans, under some protest, allowed the inspectors in two years later," a diplomat said.

During an IAEA inspection last week, South Korean officials could not produce documentation or several scientists who were involved in the work, the diplomats said. That portrayal differs significantly from those offered by U.S. officials who have repeatedly praised South Korea for coming clean voluntarily and cooperating with the IAEA.

South Korea says it has cooperated fully with the IAEA and has not been obstructionist. South Korean officials say they have produced reports for inspectors as quickly as possible given the sketchy details remaining about the 1982 plutonium experiment and the February discovery of the 2000 uranium enrichment program.

The IAEA investigation revealed South Korea's work on uranium enrichment, plutonium reprocessing and the production of nuclear equipment including uranium metal for laser technology.

When Iran was found to have been working on uranium metal, suspicions were immediately raised about its intentions. "Anytime a country makes uranium metal in secret, you have to worry that they are trying to make nuclear weapons components," said David Albright, a former IAEA nuclear inspector and the current president of the Institute for Science and International Security.

Iran was far less successful than South Korea at laser enrichment, according to diplomats and IAEA reports. In 2002, Iranian scientists enriched uranium to about 15 percent while the South Koreans, working two years earlier, enriched uranium to 77 percent, well within the range necessary for a nuclear explosive.

South Korea acknowledged the achievement in written statements to the IAEA this summer, the diplomats said. South Korean officials publicly deny uranium was enriched to high levels. The IAEA is conducting tests, and the results are expected soon.

Much of Tehran's enrichment work has been done with centrifuges, and officials there said they will continue to assemble the large-scale operation to enrich low levels of uranium for a nuclear energy program. Iran has enriched uranium to 2 percent using the centrifuges but, once mastered, the technology could be used to make highly enriched uranium suitable for bombs.

Iran's secret nuclear work was exposed two years ago, and since then IAEA inspectors have been trying to understand how and why Iran hid 18 years of effort. Iran maintains that its goal is to develop a nuclear energy program and that it worked in secret because it feared it would not be believed.

South Korea agreed, under U.S. pressure in the 1970s, to give up its nuclear weapons program. In 1991, it and North Korea agreed to ban uranium enrichment and plutonium reprocessing on the Korean Peninsula. The North is believed to have violated that agreement, and U.S. intelligence estimates indicate Pyongyang may have up to eight nuclear weapons.

In the past month, U.S. spy satellites have observed activity in North Korea that some intelligence officials believe could be a sign that Pyongyang is preparing to conduct a nuclear test, an administration official who had been briefed on the matter said last week. But he said that while the evidence, such as increased movement of vehicles at suspected test sites, was suspicious, officials were reluctant to draw firm conclusions because assumptions drawn from similar activity observed in Iraq had turned out to be wrong.

North Korea said yesterday that talks with Washington and others aimed at ending its nuclear ambitions must be tied to a full investigation of South Korea's work. Talks were to have resumed this month in Beijing, but many analysts think the next round could be delayed until after the U.S. presidential election in November.

Staff writer Glenn Kessler in Washington and correspondent Anthony Faiola in Seoul contributed to this report.

South Korean workers dismantle the facilities of a research reactor belonging to the Korea Atomic Energy Research Institute in Seoul.