In a new report on Tuesday, the International Atomic Energy Agency warned the country again to come clean over its nuclear programme.
The dossier dealt Tehran a setback in its efforts to persuade the world that its nuclear programme is peaceful and that it is fully cooperating with the UN agency.
IAEA inspectors combing Iran for evidence of a weapons programme found signs of polonium, a radioactive element that can help trigger a nuclear chain reaction, the report said.
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 had 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 programme.
Saber Zaimian, spokesman for the Atomic Energy Organisation of Iran, declined to comment on the report, saying his organisation was studying it.
The revelation came as the IAEA board prepares to convene in Vienna on 8 March 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.
Muhammad al-Baradai, the IAEA
chief, wants Iran to come clean
The IAEA's report, however, suggested the agency is more concerned with the discovery earlier this month of an advanced P-2 centrifuge system 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.
"It creates suspicions why this was not disclosed to us," a senior diplomat said, 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."
As a signatory of the Nuclear Nonproliferation Treaty, Iran is prohibited from developing nuclear weapons.
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.
"The omission... of any reference to its (Iran's)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"
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 Abd al-Qadir 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 programme 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 programme altogether. Tehran has refused to do so.