The United Nations’ nuclear watchdog said Tuesday that it has “serious concerns” that Iran is secretly working toward building a nuclear bomb, citing documents pointing to Iranian scientists’ extensive and possibly ongoing efforts to master the technology needed for atomic weapons.

The International Atomic Energy Agency cited “credible” intelligence — provided by 10 countries and vetted over many months — that directly contradicts Iran’s steadfast assertions that its nuclear intentions are entirely peaceful.

“The information indicates that Iran has carried out activities relevant to the development of a nuclear device,” the IAEA said in an uncharacteristically blunt report prepared for the U.N. agency’s 35-nation board of directors.

In its most explicit and authoritative summation of Iran’s nuclear activities to date, the U.N. agency described a structured, focused and secretive effort by Iran to acquire the essential skills for weapons-building, from warhead design to the testing of triggering devices. Although the work was halted in 2003 by order of the country’s top leaders, some key research projects appear to have been shifted to civilian institutions and “may still be ongoing,” the report said.

Iran’s official news service dismissed what it called “fake allegations” intended to further isolate the Islamic republic.

The highly anticipated report was released amid renewed threats of sanctions and even military strikes to stop Iran from building a bomb. Even as the report was being finalized in Vienna, a series of leaks from the IAEA’s intelligence dossier reinforced concerns that Iran was edging closer to nuclear-weapons capability.

Yet, despite its near-indictment of Iran for past weapons research, the IAEA report also suggested that the country is not on the brink of becoming a nuclear power. IAEA officials acknowledged uncertainty about whether Iran’s weapons research still continues, and they released fresh data suggesting continued problems with its production of enriched uranium. The output of Iran’s main uranium-enrichment plant has held steady or fallen in recent months, a phenomenon that nuclear experts attribute to aging or shoddy equipment and the lingering effects of a 2009 computer virus.

“Iran does still seem to be a long way away from having a rapid breakout capability,” said Peter Crail, a nonproliferation specialist at the Arms Control Association. Even as it gradually assembles technology for bombs, Iran “doesn’t appear to be engaged in a crash program to develop a weapon as soon as possible.”

The Obama administration, which has vowed to stop Iran from becoming a nuclear power, said the Islamic republic’s leaders have some explaining to do. “Today’s report is yet another indication of Iran’s failure to live up to its international obligations,” a senior administration official told reporters.

In Israel, Defense Minister Ehud Barak appeared to play down speculation that Israel is preparing to attack Iran’s nuclear facilities. In a radio interview before the report’s release, he stressed that Israel has “not yet” decided to take military action against Iran. He called reports of a possible strike “fear-mongering.”

At the same time, Barak de-
emphasized the cost of such a strike, saying it would not have devastating consequences for Israel. He asserted that “in certain situations we will have to rely on ourselves, and even on ourselves alone. There’s nothing new in that.”

Although the IAEA has previously confronted Iran over alleged weapons research, the agency’s new report included an unusual 14-page annex describing in sometimes minute detail how Iranian scientists pursued highly specific information, skills and materials used in nuclear warhead design. The annex was drawn from more than 1,000 pages of Iranian documents and reports that U.N. inspectors judged to be “sufficiently comprehensive and complex . . . that it is not likely to have been the result of forgery or fabrication,” the report said.

The documents enabled the IAEA to reconstruct what the report describes as a secret command structure overseeing work in technical areas ranging from uranium-metal fabrication to the design of an underground chamber where tests could be conducted. Iran appears to have procured parts and critical technical help from experts from other countries, including the assistance of a Russian weapons scientist who provided expertise on nuclear detonation, the report said.

It describes a large metal chamber allegedly built by Iran to run tests on a sophisticated detonator used to trigger a nuclear explosion. The chamber, located at a military explosives complex, was visible in satellite photos in 2000 and then shielded behind large earthen berms. IAEA officials were allowed to visit the facility in 2005 but were kept away from the area where the chamber was located.

While testing new detonators, Iranian scientists were redesigning the payload chamber of one of Iran’s longer-range missiles, the Shahab-3. “It should be noted that the dimensions of the initiation system . . . were consistent with the dimensions for the new payload,” the report said.

While more forceful than past reports, the IAEA document was cautious in drawing firm conclusions about Iran’s intentions. Still, the evidence laid out in the report “is pretty overwhelming,” said David Albright, a former U.N. nuclear inspector.

“The agency has looked at it a thousand different ways to make sure the information is credible and accurate,” Albright said. Yet for Iran, even in the face of such evidence, “it’s a very big thing to admit to.”

Correspondents Thomas Erdbrink in Tehran and Joel Greenberg in Jerusalem contributed to this report.