WASHINGTON — Iran worked for five years to develop a nuclear warhead for a ballistic missile before abruptly halting the project in late 2003, and some aspects of a nuclear weapons program “may still be ongoing,” the U.N. nuclear watchdog reported Tuesday.

It's the first time that the U.N. International Atomic Energy Agency said it believes that Iran pursued a nuclear weapon. The report to the 35-nation IAEA board of governors, however, contained no assessment of how much progress Iranian scientists may have made toward building a bomb.

“The report shows that after years of effort, the IAEA feels that Iran had a pretty large-scale program to build a warhead for a ballistic missile,” said David Albright, a former IAEA inspector who runs the Institute for Science and International Security in Washington. “We’d be in much worse shape if that program had continued.”

The report largely corroborated a 2007 U.S. intelligence estimate that said Iran had halted a nuclear weapons development program in 2003 but was keeping open the option of producing them if it decided to do so.

The long-anticipated IAEA report expanded in unprecedented depth and detail on information used in earlier reports on what the agency calls the “military dimensions” of the nuclear program that Iran hid from U.N. inspectors for 18 years and around which it continues to drape a dense veil of secrecy.

Iran denounced the report as “a mere lie,” contending that IAEA Director General Yukiya Amano, a former Japanese diplomat, relied on forged material.

The report comprises “a series of fake information added ... under U.S. political pressure,” said a commentary published by the state-run Islamic Republic News Agency.

IRNA dismissed as “metal toilets” a massive steel containment vessel that the IAEA suspects was used for nuclear weapons-related “hydrodynamic” tests of conventional explosives at Iran's Parachin military complex.

Iran long has denied having a secret nuclear weapons effort, contending that its uranium enrichment program at Natanz and near Qom is for producing low-enriched uranium for electrical power generation. Enrichment, however, also can produce highly enriched uranium used in nuclear weapons.

The report, a copy of which was obtained by McClatchy, named a succession of Iranian organizations that oversaw the project, which included the clandestine purchase of equipment and materials. It also outlined information the IAEA obtained on studies it said the Iranians had undertaken on producing a uranium bomb core and a warhead capable of being lofted atop a Shahab III ballistic missile that could hit Israel and southern Europe.

The report also said that Iran had received advice from a foreign expert, reportedly a Russian physicist; had tested sophisticated conventional explosives that can be used to compress the cores of nuclear implosion devices; had conducted computer modeling of nuclear explosions; and had made preparations for an underground test blast.

The information collected by the IAEA or provided by more than 10 member states “indicates that Iran has carried out activities relevant to the development of a nuclear explosive device. The information also indicates that prior to the end of 2003, these activities took place under a structured program, and that some activities may still be ongoing,” the report said.