The director of the U.N. nuclear watchdog agency on Monday revealed new details of South Korean nuclear experiments and efforts to conceal them during the past 20 years, casting doubt on some of the government's claims, which he described as deeply troubling.

After reports surfaced two weeks ago about secret uranium enrichment and plutonium experiments, the South Korean government said the activity was limited and was reported to the U.N. nuclear agency as required. Senior Bush administration officials have praised the U.S. ally for coming clean voluntarily and for cooperating with inspectors.

But in his first public comments since the International Atomic Energy Agency announced its investigation, Director General Mohamed ElBaradei said enrichment work "took place in three separate facilities that had not been declared to the agency," and he told the IAEA board that South Korea revealed its plutonium work only after it was confronted with evidence that inspectors had amassed over a number of years.

"It is a matter of serious concern that the conversion and enrichment of uranium and the separation of plutonium were not reported to the agency as required," ElBaradei told board members who gathered in Vienna Monday at the start of a week-long meeting.

"I would ask the Republic of Korea to continue to provide active cooperation and maximum transparency in order for the agency to gain full understanding of the extent and scope of these previously undeclared activities and to verify the correctness and completeness of South Korea's declarations," ElBaradei said.

By contrast, he gave Iran high marks for its recent cooperation and said several key issues regarding the country's nuclear work had been clarified. But those positive steps may be jeopardized, ElBaradei warned, if Iran continues to build equipment that could be used for bomb-making.

"I have continued to stress to Iran that during this delicate phase, while work is still in progress to verify its past nuclear program, and in light of serious international concerns surrounding the program, it should do its utmost to build a required confidence through the agency," he said.

His mix of praise and criticism came as the Bush administration was struggling to make diplomatic headway on Iran and the Korean Peninsula. On Monday, U.S. and European diplomats shuttled between conference rooms at the IAEA meeting in an effort to bridge differences over how to deal with Iran. The Bush administration wants a resolution setting a deadline for Iran to fully suspend all suspect nuclear work or face U.N. Security Council measures. European diplomats, holding out the possibility that inspectors could answer any remaining questions by then, prefer to await the outcome of the IAEA inspections.

Iran's lead negotiator here, Hoseyn Moussavian, said Iran has halted work, for the moment, on the construction of a large-scale centrifuge operation that could be used to enrich uranium. "We have taken maximum steps for confidence-building," he told reporters in Vienna. ElBaradei dispatched inspectors to Iran several days ago to tag equipment but it remains unclear under what terms the Iranians would continue to hold off on the work.

ElBaradei said that negotiations are at a sensitive stage and that he has been consulting with all parties. He said he could not set a timeline for completing the Iran investigation but said he wanted remaining issues to be clarified by the end of the year.

He did not say whether Iran should be reported to the U.N. Security Council for violations of the nuclear Non-Proliferation Treaty. But diplomats at the IAEA said South Korea was increasingly facing a referral to the council.

ElBaradei also provided information about the nuclear operations uncovered in South Korea, including experiments in plutonium conducted in the early 1980s, production of nuclear equipment in the mid-1990s and remarkably successful uranium enrichment work in 2000.

Both Iran and South Korea are signatories of the international nuclear treaty that the IAEA was set up to help implement. In the 1970s, the South Korean government announced it was giving up its nuclear weapons program under U.S. pressure. In 1991, it signed an agreement with North Korea 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 that North Korea may have as many as eight nuclear weapons.

An explosion last week in North Korea set off alarm that it may have conducted a nuclear test. On Monday, North Korean officials told a visiting British diplomat in Pyongyang that the explosion and resultant billowing cloud of smoke was caused by the planned demolition of a mountain for a hydroelectric project, Western diplomats said.

The North Korean government said over the weekend that six-nation talks aimed at ending its nuclear ambitions must be tied to a full investigation of South Korea's nuclear work.

[On Tuesday, after leaving Pyongyang, the British official told reporters that North Korea was committed to holding such talks, but was not prepared to set a date, the Reuters news service reported from Beijing.]

Mohamed ElBaradei, the head of the IAEA, said unreported nuclear efforts in South Korea were "a matter of serious concern."