Security Council Secretary Igor Ivanov, speaking to Russia's ITAR-TASS news agency on a visit to Tehran, said Russian specialists are actively working at the Bushehr facility in southern Iran.
The $800 million Russian-built reactor is scheduled to come on line by the end of next year.
In another development, Iran has rejected a compromise proposal that could have transferred Iranian uranium enrichment activities out of Iran to Russia.
The head of Iran's atomic energy program, Gholamreza Aghazadeh, on 12 November said Iranian nuclear fuel must be produced in Iran.
The United Nations' nuclear watchdog agency on 24 November is due to discuss whether to refer Iran to the UN Security Council for possible sanctions connected to its nuclear program.