The US government, which has spearheaded toughening rounds of sanctions against Tehran, added it was studying what "diplomatic next step" to take after Monday's report from the International Atomic Energy Agency (IAEA).

France and Germany -- which along with Britain, Russia and China are locked in carrot-and-stick negotiations with Iran over its nuclear program -- voiced immediate concerns about the report.

The IAEA on Monday expressed "serious concern" that Iran is still hiding information about alleged studies into making nuclear warheads and defying UN demands to suspend uranium enrichment.

The alleged studies comprise a uranium conversion project, high explosives testing and designs for a missile re-entry vehicle, and suggest there may have been a possible military angle to past nuclear work.

The report "refers to the fact that the Iranians are willfully -- trying to willfully withhold information about their activities related to potential weaponization," McCormack told reporters in Washington.

"There are a number of different questions out there about the military's involvement in this nuclear program, about Iran's efforts to fabricate hemispheres of uranium," he said.

"And I'm not sure other than for a weapon why you would do that," McCormack added.

Washington has in the past pointed to IAEA inquiries into Iran's possession of blueprints for making the uranium metal hemispheres that are the core of bombs as signs of military intentions.

McCormack called the new report "very troubling" and "disturbing," adding: "And we'll see what diplomatic next step will flow from this."

The State Department spokesman said the report will have no impact on a so-called "refreshed" package of incentives for Iran to halt its uranium enrichment and reprocessing work.

"The package is set," McCormack said.

European Union foreign policy chief Javier Solana said he hopes to travel to Iran soon to present the package from the six negotiating powers after meeting Tehran's Foreign Minister Manouchehr Mottaki over the weekend.

The permanent Security Council members Britain, China, France, Russia and the United States plus Germany have re-worked an offer of political and trade incentives originally made in 2006.

Solana has been struggling to establish high-level talks aimed at getting Iran to accept the package but Tehran refuses to suspend enrichment as a precondition for negotiating.

Iran insists it has the right to enrich uranium to make nuclear fuel to help meet its electricity needs and has so far defied three sets of Security Council resolutions which demand it halt the process.

At highly refined levels, such work can also make the fissile core of an atomic bomb but Iran says its nuclear program is peaceful and has vehemently denied that it is seeking to make weapons with it.

In Paris, French foreign ministry spokeswoman Pascale Andreani said the details mentioned in the report "could be the sign of a possible military dimension of the Iranian nuclear program."

In Berlin, German Foreign Minister Frank-Walter Steinmeier said the report "leaves open a number of questions that we will have to examine very quickly."

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