Russia has signed a contract to build two more nuclear reactors in Iran likely to be followed with another six, a move intended to cement closer ties between the two nations.
The deal comes less than two weeks ahead of the November 24 deadline for Tehran to sign an agreement on its nuclear programme with six world powers.
Tuesday's contract has no immediate relation to the talks that involve Russia and the United States, but it reflects Moscow's intention to deepen its co-operation with Tehran ahead of possible softening of Western sanctions against Iran.
The nuclear officials from the two countries signed the contract for building two reactors at Iran's first Russia-built nuclear plant in Bushehr.
Sergei Kiriyenko, the head of Russia's Rosatom state corporation, and Iran's nuclear chief Ali Akbar Salehi also signed a protocol envisaging possible construction of two more reactors in Bushehr and another four in an undetermined location.
"It's a turning point in the development of relations between our countries," Salehi said after the signing, according to Russian news reports.
Rosatom said in a statement that the construction of the new reactors will be monitored by the International Atomic Energy Agency.
As in the case of Bushehr's first reactor that became operational in 2013, Russia will supply uranium fuel and then take it back for reprocessing - a provision intended to prevent a possibility of Iran using the spent fuel to build atomic weapons.
A potential agreement between Iran and the six powers would ease Western sanctions against Iran's economy if Tehran agrees to limit its uranium enrichment to a level that would make it unable to build nuclear weapons.
Iran has dismissed Western suspicions that it was working covertly to develop nuclear weapons, insisting that its nuclear activities are aimed at peaceful energy demands and medical needs.