India and Russia have traditionally enjoyed good relations
Visiting Russian President Dmitry Medvedev has signed a key deal to build four nuclear power plants in India.
President Medvedev signed the accord in the Indian capital, Delhi, with Prime Minister Manmohan Singh.
The deal follows the landmark civilian nuclear accord between India and United States earlier this year.
In September, the Nuclear Suppliers Group lifted a ban that had stopped India from getting access to the global nuclear market.
The Russian agreement is part of a series of deals, including ones on space and defence sales.
Russia will now build four nuclear energy reactors at Kudankulam in the southern Indian state of Tamil Nadu.
Russia is already building two other reactors at the Kudankulam site.
Russia becomes the third country to sign a nuclear deal with India after the signing of the India-US agreement which allows India access to civilian nuclear technology and fuel.
France has also signed a co-operation pact with Delhi.
Moscow and Delhi also signed a deal under which Russia will assist India in its space programme, including sending Indian astronauts into space.
And India will buy 80 military helicopters from Russia, cementing a relationship that dates back to the Cold War.
President Medvedev also pledged to support India's fight against terrorism following last week's Mumbai attacks.
The Russian president is on a three-day visit. On his arrival in Delhi, he was welcomed with a full military salute at the presidential palace.
Later, he visited the memorial to Mahatma Gandhi, the father of the Indian nation.
India and Russia have been traditional allies and around 70% of India's military hardware comes from Russia.