U.N. inspectors in Iran have uncovered evidence of nuclear experiments that Tehran did not previously disclose, the International Atomic Energy Agency (search) said Tuesday in a report warning the country anew to come clean.

The dossier, a copy of which was obtained by The Associated Press, dealt the Tehran regime a setback in its efforts to convince the world that its nuclear program is peaceful and that it is fully cooperating with the U.N. agency.

IAEA inspectors combing Iran for evidence of a weapons program found signs of polonium (search), a radioactive element that can help trigger a nuclear chain reaction, the report said. It was distributed to the agency's 35-nation board of governors ahead of a key meeting on Iran early next month.

The agency said the traces of polonium-210 were found in September, and that the element "could be used for military purposes ... specifically as a neutron initiator in some designs of nuclear weapons."

Iran never mentioned working with polonium-210 in earlier declarations of its past and present nuclear activities, it said.

Polonium-210 also can be used to generate electricity, which Iran contends is the sole purpose of its atomic program. Saber Zaimian, spokesman for the Atomic Energy Organization of Iran, declined to comment on the report, saying his organization was studying it.

The revelation came as the IAEA board prepares to convene in Vienna on March 8 to reassess the Iranian threat amid mounting pressure from the United States and other countries that contend Iran has been trying to build an atomic bomb.

The report suggested the agency is more concerned with the discovery earlier this month of an advanced P-2 centrifuge system (search) in Iran that could enrich uranium for weapons use. The Bush administration, too, has said the finding raises "serious concerns" about Tehran's intentions.

"The omission ... of any reference to its possession of the P-2 centrifuge design drawings and associated research, manufacturing and mechanical testing activities is a matter of serious concern, particularly in view of the importance and sensitivity of those activities," the IAEA report said.

"We have seen some good cooperation from Iran, particularly in regard to access to sites," IAEA Director-General Mohamed ElBaradei told reporters en route back from a visit to Libya on Tuesday. "I would like to see, however, much more prompt information coming from Iran."

ElBaradei called the P-2 centrifuge discovery a "setback," adding: "I hope this will be the last time any aspect of the program has not been declared to us."

"It creates suspicions why this was not disclosed to us," a senior diplomat told AP, speaking on condition of anonymity. "They said it was a full and final declaration. The question is -- is there something else to be declared? We are trying to create confidence. This is a real setback."

The agency said Tehran has assured the IAEA it will suspend the assembly and testing of centrifuges and the manufacture of centrifuge components by next week. It called on Iran to give a "correct and complete" accounting of its nuclear activities, but said the government was "actively cooperating" with the agency.

"As a result of its monitoring activities, the agency is able to confirm that there has been no operation or testing of any centrifuges, either with or without nuclear material," at Iran's pilot fuel enrichment plant, Tuesday's report said.

Confronted by evidence last year, Iran acknowledged hiding nearly two decades of nuclear activity, including importing enrichment technology linked to the black market network of Pakistani scientist Abdul Qadeer Khan.

Those imports of equipment and expertise have allowed Tehran to create a domestic production line of centrifuges that can be used both to process uranium for power -- or enrich it to levels high enough to manufacture warheads.

Under international pressure last year, Iran pledged to cooperate fully with the IAEA in efforts to prove it was not interested in nuclear weapons, including opening its activities to full outside scrutiny.

Iran suspended its enrichment program last year but continues to make and assemble centrifuges despite international criticism that such actions violate the spirit of its pledge to stop all enrichment activities.

The IAEA, along with the United States and other nations, wants Iran to scrap its enrichment program altogether. Tehran has refused to do so.