WASHINGTON — Iran is believed to be withholding information on alleged studies it conducted as part of a secret nuclear warhead development project, a new U.N. International Atomic Energy Agency report said Monday.

The new report also indicated that Iran has become significantly more proficient at enriching uranium, in defiance of U.N. Security Council demands that it suspend its enrichment program, doubling its production and apparently conquering previous technical problems with its enrichment equipment.

Taken together, the conclusions suggest that Iran has made few steps to quiet 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 charges 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 June 2.

"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 U.S. 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 are "baseless" and "fabricated" and repeated that it does not seek to develop nuclear weapons.

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 hexafloride 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 further identified because he was not authorized to speak to reporters, said the increased low-enriched uranium production indicates that Iranian centrifuges "are working a lot better than they had been" and are now "operating in a stable manner."

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