Iran has started loading fuel into the core of its first nuclear power plant, according to state television.
Fuel rods were transported into the southern Bushehr reactor building in August but work was delayed. Iran says it will now start producing energy early next year.
Tehran continues to insist its nuclear ambitions are peaceful. Some countries fear it really wants to develop nuclear weapons.
Iran is showing no sign of backing down in the dispute over its separate uranium enrichment programme despite UN sanctions.
Experts say firing up the Bushehr plant will not take Iran any closer to building a nuclear bomb. That is because Russia will supply enriched uranium for the reactor and take away the spent fuel which could be used to make weapons-grade plutonium.