Iran had to unload fuel due to safety concerns in February. Russia said a breakdown in one the reactor's cooling pumps necessitated the removal of 163 fuel rods from its core.

"The loading of fuel-rod assemblies into the core started at the Bushehr Nuclear Power Plant on April 8," Atomstroyexport, which specializes on the construction of nuclear power facilities abroad, said in a statement.

The fuel for Bushehr was provided by Russia, which completed the long-delayed construction of the plant last summer. The plant was expected to start feeding electricity into the national grid in February.

Under a bilateral agreement, which has received approval from the International Atomic Energy Agency, Russia will initially operate the plant, supplying its fuel and taking away all the spent fuel for the next two or three years, but will eventually hand over full control to Iran.