The International Atomic Energy Agency faulted Iran yesterday for failing to reveal the extent of its nuclear research program, months after the Tehran government had promised to tell all.

Iran failed to inform the U.N. nuclear watchdog agency that it possessed an advanced design for gas centrifuges and had produced and tested components. The explanation for the omission is "difficult to comprehend," the IAEA said in a report circulated in Vienna.

The presence of traces of uranium on enrichment equipment leaves "discrepancies and unanswered questions," said the IAEA, which also asserted that Iran has not substantiated its benign explanation for newly discovered tests on polonium, an element valuable in atomic weapons design.

The report, prepared in advance of a March 8 IAEA board meeting, ensures that Iran will face continued skepticism about its pledge to remain free of nuclear arms. Any hope in Tehran that the country would soon be freed from close IAEA scrutiny is dead, analysts said.

"It's a very incriminating report. The Iranians are still not in an acceptable position," said Robert Einhorn, a Clinton administration nuclear specialist now at the Center for Strategic and International Studies.

Iranian authorities, briefed in advance by IAEA Director General Mohamed ElBaradei, pledged within hours of the report's distribution to take stronger steps to suspend their uranium-enrichment program, a demand made last year by the agency and renewed by European foreign ministers who offered improved trade in return.

Iran, which earlier shut down its enrichment facility but continued to assemble gas centrifuges, declared yesterday that it will stop the assembly and testing of centrifuges. It did not promise to halt all domestic production of components but said any new parts would be placed under IAEA seal.

The IAEA welcomed the move and credited Iran with "actively cooperating" with inspectors who want to visit specific sites. But ElBaradei, the report said, has also requested that Iran "intensify" its cooperation, particularly through the "prompt provision of detailed information."

The Bush administration, which has remained openly doubtful of Iran's intentions, praised the toughness of the report and said it will continue to work within the IAEA to press Iran for results, a State Department official said.

"Iran's hand was caught in the cookie jar. The only way out now is to own up to what they did, just as Libya did," said another U.S. official involved in nuclear policymaking who spoke on the condition of anonymity. "Added together, these activities can only mean a weapons program. They're not just making isotopes for hospital use."

White House officials are hoping to use the upcoming IAEA board meeting to highlight the contrast between Iran and Libya, which agreed in December to renounce weapons of mass destruction.

The 13-page IAEA dossier is a report card on Iran's compliance with a safeguards agreement required by the 1970 nuclear Non-Proliferation Treaty and follows a deeply critical report in November that revealed a program hidden for 18 years. The new document cites progress, but it says progress has been undermined by explanations the agency considers dubious.

A key question concerns the contamination of centrifuge components produced in Iran. While Iran has said the uranium traces originated on equipment imported from Pakistan, agency tests show the contamination on the imported and local components to be different, suggesting that Iran may have enriched uranium on its own.

Also, the uranium traces differ in type at two Iranian sites -- the Kalaye Electric Co. and the Natanz enrichment facility -- even though Iran said the source of contamination in both places was the Pakistan equipment.

Finally, samples taken from one room at Kalaye showed the presence of uranium enriched to 25 times the level acknowledged by Iran.

IAEA officials were troubled by Iran's failure to reveal the existence of more advanced P-2 centrifuge designs and components despite an October declaration that ostensibly detailed its entire program. Iran offered a new series of explanations this month, which the IAEA found perplexing.

The discovery of polonium in records dating to 1989-93 also raised inspectors' suspicions.

Iran said the intensely radioactive substance was intended as a power source, adding later that tests were also conducted for commercial reasons.

"Polonium's most plausible use in Iran is in nuclear weapons cells," Einhorn said. "The cover story that it was for nuclear batteries just doesn't stand scrutiny."

To some observers, the timing of Iran's promise to halt centrifuge production suggested that Iran hoped to soften the impact of a negative report.

"In every case, Iran has only made revelations when it knew it was about to be confronted with evidence," said Michael A. Levi, a Brookings Institution nonproliferation expert. "By delaying the decision until the last minute, Iran makes it very difficult for the United States, the Europeans and others to come up with a common approach."