Iran says it has reached a basic agreement with Russia on uranium enrichment.
However officials are not giving details of the accord, or revealing whether Tehran is now willing to move sensitive parts of its nuclear programme to Russian soil.
So far, Iran has insisted it has the right to reprocess uranium on its own territory.
Sergei Kirienko, the boss of Russia’s atomic energy agency, said talks would continue in the coming days.
He visited the site at Bushehr, the focus of much controversy in the West. The reactor is being built with Moscow’s help and is due to come on stream this year.
The EU and the US worry it could be used as a cover to make material for nuclear weapons.
Kirienko said the plant’s opening would be slightly delayed for technicalreasons and not because of political concerns.