Russia and Iran signed a nuclear fuel supply deal yesterday to pave the way for Iran to start up its first atomic reactor next year.
The agreement, inked by the two countries' nuclear energy chiefs at the Bushehr atomic plant in southern Iran, came as Tehran faced heightened pressure from the United States, which accuses it of secretly developing nuclear weapons.
The agreement has long been opposed by Washington.
Iran, OPEC's second-largest oil producer, denies the charge and has received strong backing from Moscow, which is keen to play a major role in expanding Iran's nuclear energy program.
"This is a very important incident in the ties between the two countries and in the near future a number of Russian experts will be sent to Bushehr to equip the power station," Iranian state television quoted Mr Alexander Rumyantsev, head of Russia's Federal Atomic Energy Agency, as saying.
A key part of the agreement obliges Tehran to repatriate all spent nuclear fuel to Russia. Moscow hopes this will allay US worries that Iran may use the spent fuel, which could be reprocessed into bomb-grade plutonium, to develop arms.
The International Atomic Energy Agency, which has been investigating Iran's nuclear programme for over two years, said it would also keep a careful eye on Tehran's use of the fuel.
The White House had no immediate comment on the signing of the supply agreement.