The United Nations nuclear agency says Iran is planning to test a facility that could convert raw uranium into weapons-grade material.
Iran denies it wants to build nuclear weapons
But the International Atomic Energy Agency report did not confirm whether Iran is trying to build a nuclear bomb.
Iran insists the only purpose of its nuclear programme is power generation.
Tehran welcomed the report, which it said was a positive step towards demonstrating the peaceful nature of its nuclear project.
The IAEA document said Iran wants to turn 37 tonnes of raw "yellow cake" uranium into uranium hexafluoride.
Key issues remain
That is spun in centrifuges to produce enriched uranium, which can be used to generate power or for nuclear warheads, depending on the level of enrichment.
But the report said that Iran did not appear to be operating or installing centrifuges.
A Western diplomat told the BBC that the move was a clear sign that Iran intends to resume its enrichment programme at some stage.
The report says two issues remain key to understanding the nature of Tehran's enrichment programme.
Those are the origin of uranium contamination at various locations in the country and the extent of its efforts to import, build and use advanced centrifuges.
The United States believes Iran could be just a few years away from making a nuclear bomb.