Iran has rapidly ramped up production of higher-grade enriched uranium over the last four months, the UN nuclear agency says in a confidential report.
The report fuels fears about how quickly the Islamic Republic could produce an atomic bomb.
The International Atomic Energy Agency also said Iran had failed to give a convincing explanation about a quantity of missing uranium metal.
Diplomats have said the missing amount could be used for experiments used to arm a warhead.
Iran insists it is not interested in nuclear weapons and says all of its activities are meant either to generate energy or to be used for research.
But the report expresses "serious concerns regarding possible military dimensions to Iran's nuclear programme."