Russia says it will undertake a key step next week towards starting up a reactor at Iran's first nuclear power station.
Russia's state atomic corporation, which is building the plant, said engineers will begin loading the Bushehr reactor with fuel.
However, it could be six months before the reactor is fully operational.
Russia has been helping build the plant since the mid-1990s, amid tensions over Iran's nuclear programme.
"The fuel will be charged in the reactor on 21 August. From this moment, Bushehr will be considered a nuclear installation," spokesman Sergei Novikov said.
Iranians will remain sceptical until they see the Bushehr plant finally working and generating electricity, 35 years after the project was started under the Shah, says the BBC's Jon Leyne in Cairo.
Many in Iran believe that the endless delays in the civilian project were designed either to extract more money from them, or as a result of Western pressure.
If and when the plant finally starts operating, it will be a moment of national pride, and an event Iran will no doubt celebrate as showing it can overcome international pressure and isolation, our correspondent says.
Russia will run the plant, supply the fuel and take away the fuel waste.
For that reason, nuclear experts say there is little immediate danger of the reactor being used to build nuclear weapons.
Russia is a leading arms and technology supplier to Iran, though relations have cooled in recent months as Moscow has backed tougher UN sanctions aimed at pressuring Iran to abandon its uranium enrichment programme.
Moscow's co-operation with the sanctions is seen as key to their effectiveness.