WASHINGTON - Iran is believed to be withholding information on studies it allegedly conducted as part of a secret project to develop nuclear warheads, a new U.N. International Atomic Energy Agency report said yesterday.

The report also indicated that Iran had become significantly more proficient at enriching uranium - in defiance of U.N. Security Council demands that it suspend its enrichment program. The report suggested that Iran had doubled its production and apparently had overcome previous technical problems with its enrichment equipment.

Taken together, the conclusions suggest that Iran has made few steps to ease international anxiety over its nuclear program.

"The alleged studies on the green salt project, high explosives testing, and missile re-entry vehicle project remain a matter of serious concern," said the confidential report, a copy of which was obtained by McClatchy. "Clarification of this is critical to an assessment of Iran's past and present nuclear program."

The Bush administration, which has recently stepped up allegations that Iran is sowing instability across the Middle East and arming Shiite Muslim militias in Iraq, is likely to seize on the report to bolster its assertion that the theocratic regime in Tehran is bent on developing nuclear weapons.

The nine-page IAEA report is to be presented to the agency's 35-nation Board of Governors on Monday.

"The agency is of the view that Iran may have additional information, in particular on high explosives testing and missile-related activities, which could shed more light on the nature of these alleged studies and which Iran should share with the agency," the report said. "It is essential that Iran provide all requested information, clarifications and access outlined in this report without further delay."

The report said Iran must also provide information on the roles that military-run research facilities and industries have played in the country's nuclear program, including in fabricating and procuring components.

The information outlining the alleged studies was provided to the IAEA by the United States and other countries. It reportedly shows links between conventional explosives testing, uranium hexafluoride production, and modifications that would allow a Shahab-3 missile to carry a nuclear warhead.

Iran, the report said, agreed to provide a response to the data, but insisted that the materials were "baseless" and "fabricated," and repeated that it does not seek to develop nuclear weapons.

Ali Ashgar Soltanieh, Iran's chief delegate to the IAEA, said the report described "the peaceful nature of our nuclear actions." But Gregory L. Schulte, his U.S. counterpart, said the report was a strong indictment of Iran's defiance of the international community's efforts to get answers on its nuclear program.

Tehran has refused to suspend its enrichment program despite three rounds of U.N. Security Council sanctions that U.S. and European officials say are adding to the country's economic woes by drying up its access to hard currency.

The report also provided an update on Iran's progress in developing the ability to enrich uranium, a process that involves spinning uranium hexafluoride gas in thousands of machines known as centrifuges. The process can produce low-enriched uranium, for use in electrical generating plants, or highly enriched uranium, used for nuclear weapons.

Iran, the report said, made 150 kilograms of low-enriched uranium at its industrial-scale plant at Natanz, which is under IAEA monitoring, in the first five months of this year, about double the amount produced between February and December last year.

A senior U.N. official close to the IAEA, who asked not to be identified because he was not authorized to speak to reporters, said the increased low-enriched uranium production indicated that Iranian centrifuges "are working a lot better than they had been."

Still, the report said Iranian officials may be exaggerating about their progress. The report said Iran had installed about 500 new centrifuges since February, many fewer than the 6,000 centrifuges that Iranian President Mahmoud Ahmadinejad asserted in April had been installed.