Russia and Iran have signed an agreement for Moscow to supply fuel to Iran's new nuclear reactor in Bushehr.
The signing had been delayed several times
Under the deal Iran has to return spent nuclear fuel rods from the reactor, which was designed and built by Russia.
The clause is a safeguard meant to banish fears that Iran might misuse the rods to build nuclear weapons, a concern of the US, Israel and others.
The agreement sets out a time-frame for delivery of the fuel, but officials said the dates would be kept secret.
The BBC's Frances Harrison in Tehran says the deal is significant because Bushehr will be Iran's first reactor to go on stream - a project that has become an issue of national prestige in the face of intense US opposition.
The signing, which had been expected on Saturday, was apparently delayed over disagreements about when the spent fuel should be returned.
Russia had been insisting that no spent fuel should be diverted for the manufacture of weapons.
Iran has repeatedly said its nuclear programme is solely for the generation of power.
'Strongest indication yet'
But despite Iran's denials, diplomats said investigations showed Tehran had had full possession of enrichment know-how for two decades, after acquiring the information from Pakistani nuclear scientist AQ Khan's black-market network.
Iran says its nuclear activity has solely peaceful motives
The Washington Post on Sunday quoted officials as saying that the programme originated at a secret meeting in Dubai 18 years ago between Iranian officials and Mr Khan's associates.
The officials said Tehran, which was then at war with Iraq, bought centrifuges and an enrichment starter kit but also used the meeting as a guide before purchasing more expensive items elsewhere.
The offer "was the strongest indication to date" that Iran had a nuclear weapons programme, a diplomat was quoted as saying.
Iran's nuclear energy chief Gholamreza Aghazadeh and his Russian counterpart, Alexander Rumyantsev, agreed the deal.
"We have signed a confidential protocol that sets out the timetable for the delivery of fuel to the nuclear power plant at Bushehr," Mr Rumyantsev said, quoted by Russian news agency Itar-Tass.
Russia has rejected US pressure to cut nuclear co-operation with Iran.
Washington is also concerned that the nuclear project could allow for the covert transfer of weapons technology to Iran.
At a meeting between Russian President Vladimir Putin and US President George W Bush in Slovakia on Thursday, both sides agreed Iran should not be allowed to develop nuclear weapons.
The signing of the nuclear fuel deal has been delayed several times, apparently over technical and financial issues.
Our correspondent says Iran is currently in negotiations over its nuclear programme with Europe, and one of the incentives on the table is an offer of a nuclear reactor from the West.
If Iran is already receiving nuclear fuel and technology from Russia, she says, it is in a stronger negotiating position.