Iran has carried out tests specific to "the development of a nuclear device", the UN's atomic watchdog has reported.
The International Atomic Energy Agency (IAEA) report highlights research into "computer models that could only be used to develop a nuclear bomb".
The report says: "The application of such studies to anything other than nuclear explosives is unclear to the agency."
Iran publically gave up its weapons programme in 2003, and has maintained that the country's nuclear research programme is purely for the purpose of generating civilian power.
However, the IAEA and western governments have long-suspected that the programme to turn Iran into a nuclear-armed state continued in secrecy, with Wednesday's publication likely to ramp up tensions in the region.