The UN atomic agency revealed that Iran received black-market designs to encase weapons-grade uranium and diplomats said they appeared to be part of blueprints for a nuclear warhead.
A senior US diplomat called the find disturbing and other diplomats accredited to the International Atomic Energy Agency said they expected the United States and its allies to use it in their push to have Tehran referred to the UN Security Council as early as next week.
"You've given the world cause for concern," US State Department spokesman Adam Ereli said in Washington. "The international community doesn't like what it sees."
The revelations came Friday as Iran said it had begun converting a second batch of uranium into gas, a step that brings it closer to producing the enriched uranium used to either generate electricity or build bombs.
The European Union, with US support, has been calling on Iran to re-impose a freeze on conversion since August. But the nation's top nuclear negotiator, Ali Larijani, told state TV the country had started converting a second batch of uranium.