Russia has signed a preliminary agreement to build at least two more nuclear power plants in the Iranian port city of Bushehr, Iran's official IRNA news agency reported on Wednesday.
The deal was reached during a visit to Tehran on Tuesday by a senior official of Russia's state atomic energy agency Rosatom, IRNA said.
"Iran and Russia reached a preliminary agreement to build at least two new nuclear power plants," Iranian Atomic Energy Organisation spokesman Behrouz Kamalvandi told the news agency.
The two new 1,000 megawatt plants will be constructed alongside the existing power station in Bushehr, which was also built by Russia, Kamalvandi said.
Further talks will be held on technical and financial aspects of the project, but a final agreement is expected to be signed "very soon", he added.
In January, Iranian President Hassan Rouhani told his Russian counterpart Vladimir Putin that establishing long-term relations between the two countries "can serve the stability and security of the region".
Iranian media last month speculated that Rouhani could travel to Russia for a regional conference of Caspian Sea states that Tehran's envoy to Moscow has said will be in late September.
Construction of the new Bushehr nuclear plants is likely to spark concerns among Gulf Arab states, which have often raised concerns about the reliability of the existing Bushehr facility and the risk of radioactive leaks in case of a major earthquake.
Iran sits astride several major fault lines and is prone to frequent quakes. On April 9, a 6.1-magnitude quake rocked the south, with an epicentre just 100 kilometres (60 miles) from Bushehr.
Both Iran and Russia have dismissed the claim, saying the Bushehr facility is subject to inspection by and the safeguards of the International Atomic Energy Agency (IAEA), the UN watchdog.
Reducing reliance on oil
Western powers and Israel suspect the Islamic republic's nuclear programme masks a covert weapons drive. Tehran denies the charge, saying that by diversifying its energy resources it wants to reduce its reliance on oil revenues.
Iran, which still faces tight Western sanctions on its oil and banking sectors despite a landmark agreement reached with major powers in November on its nuclear programme, is expected to fund the new Bushehr project on a barter basis.
Tehran's ambassador to Russia, Mehdi Sanaei, said last month that the close trading partners have been negotiating the delivery by Iran of hundreds of thousands of barrels of oil per day (bpd) in return for Russian goods and services, including the planned new nuclear plants.
Russian officials have neither confirmed nor denied the discussions, while stressing that they would not break UN sanctions.
But the White House has raised "serious concern" about the potential of the mooted deal to undermine EU and US sanctions which it credits with bringing Iran to the nuclear negotiating table.
One Russian report said the barter agreement could see as much as 500,000 bpd of Iranian crude exchanged for Russian goods, which Sanaei said could also include heavy trucks and railway equipment.
That would represent a boost of more than 50 percent to Iran's crude exports, undermining the crippling sanctions that Western governments credit with securing its signature to the long-sought interim nuclear deal agreed in November.
Iran and world powers are still negotiating a long-term agreement to allay Western concerns about its nuclear ambitions.