The two states have been working for more than a year on a deal that will allow Russia build the reactors at the Kudankulam nuclear power plant in Tamil Nadu.

"An agreement on building additional reactors at the Kudankulam atomic station has been initialised," Deputy Prime Minister Alexander Zhukov said in New Delhi.

Zhukov said the initialisation -- which means officials signed a preliminary deal -- had taken place just before a visit to India by Prime Minister Viktor Zubkov.

The deal cannot be finalised because of restrictions on India imposed by the 45-nation Nuclear Suppliers Group (NSG) and the limits are unlikely to be lifted until a US-India deal on civilian nuclear cooperation is ratified.

Russia is competing with the United States for influence in India, a Cold War ally of Moscow which the Kremlin sees as a growing partner in Asia.