Diplomatic sources say the International Atomic Energy Agency has found sophisticated uranium-enrichment equipment in Iran.
Diplomats, who spoke on condition of anonymity, said the gas centrifuge system found is capable of processing uranium that can be used both as nuclear fuel and for warheads, depending on the level of enrichment.
The IAEA, whose inspectors are in Iran to find out if the country was secretly trying to develop nuclear weapons, declined comment.
Last week, diplomats said IAEA inspectors had found drawings of an advanced centrifuge design that Iran did not acknowledge having, despite pledging to be fully open about its nuclear activities.
Diplomats say the latest find at an air force base outside Tehran puts in doubt Iran's claim that its military was not involved in the country's nuclear program.
Iran insists its nuclear program is for peaceful purposes only. Last October, Tehran agreed to allow surprise inspections of its atomic facilities and to temporarily halt its uranium enrichment program.
Information for this report is provided by AP, AFP and Reuters.