The International Atomic Energy Agency (IAEA) says in a new report that Iran has begun installing more advanced centrifuges at its Natanz uranium enrichment plant.
Tehran says it needs refined uranium to fuel nuclear power plants. Western powers fear it is seeking to produce material for nuclear weapons.
The IAEA report says Iran has so far produced 280 kilograms of 20-percent enriched uranium, of which around 110 kilograms have been converted into fuel.
Experts say that around 250 kilograms are needed for one bomb, if enriched to a higher degree.
The IAEA report on February 21 comes ahead of talks between Iran and six major powers -- Britain, China, France, Germany, Russia, and the United States -- on Tehran's nuclear program.
The February 26 talks in Almaty, Kazakhstan will be the first after a gap of eight months
Based on reporting by Reuters, dpa, and AFP