The report says among other things that Iran is preparing to refine some 37 tons of yellowcake uranium into a concentrated form that could provide the material for several nuclear weapons.

The report was circulated Wednesday to member countries of the governing board of the International Atomic Energy Agency, the IAEA, which is to begin a crucial meeting on the Iranian nuclear program in Vienna September 13.

While Iran insists that its nuclear program is entirely peaceful, the United States has long held that Tehran has a clandestine nuclear weapons program.

The report by IAEA inspectors, made available to news media, says they have not uncovered definitive evidence, one way or the other, about a bomb program. But it also says they have been unable to clear up suspicions and unanswered questions about Iran's activities.

In a written statement here, the Bush administration's chief arms control official, Under-Secretary of State John Bolton, said findings in the report are of great concern and constitute further strong evidence of the compelling need to take Iran's nuclear program to the U.N. Security Council.

Mr. Bolton said the United States will continue to urge other IAEA board members to join in the effort to deal with what he termed the Iranian threat to international peace and security.

The 35-nation IAEA board could vote to refer the matter to the Security Council for possible economic sanctions against Iran, but the United States has not said whether it will seek such a referral at the next meeting.

At a news briefing, State Department spokesman Thomas Casey said the report shows Iran's failure to come clean with the IAEA and that the United States looks forward to discussing with other board members what the next steps should be.

"I think we want to have a chance to talk with our partners in the IAEA. I think the board needs to decide what the next steps are," he said. "Clearly there is a pattern of deception, a pattern of non-compliance, and the board needs to decide in light of those failures, what it ought to be doing next."

Iran's foreign ministry acknowledged Wednesday the IAEA report left some questions unanswered but said they are insignificant.

It also said that some countries were trying to make a fuss about the findings in order to create what it termed a negative atmosphere about Iran.

Iran has previously threatened to sever cooperation with the IAEA if the governing board moved for sanctions against it.