Sergei Ivanov, the Russian deputy prime minister, said the reactor agreement covered the construction of "up to 16 nuclear energy units" at three Indian sites.
Two units are already under construction in the southern Indian state of Tamil Nadu and Russia won a deal to build four more in 2008.
It was unclear if the 16 reactors referred to by Ivanov included these six.
Uday Bhaskar, a New Delhi-based defence and security analyst, told Al Jazeera that Putin's visit was significant in many ways.
"This trip is about consolidating their relationship. There are a number of defence and strategic deals that will be signed. I also think there will be some political issues talked about such as how to deal with Afghanistan and also the rise of China," he said.
Russian officials had said Putin would sign more than a dozen pacts amounting to $10bn.
One of the accords signed on Friday aims to resolve the troubled sale of a refitted Soviet-era aircraft carrier, the Admiral Gorshkov.
Price disputes and delayed deliveries in the way of the sale of the Admiral Gorshkov had sparking concerns in Moscow that India may be planning to end its dependence on Russian military equipment.
Ivanov, the Russian deputy prime minister, said the vessel would be delivered by the end of 2012. The final cost was not revealed, although experts believe it to be around $2.3bn.
On the other hand, Mikhail Pogosyan, the general director of the Russian aircraft manufacturer RAC-MiG, estimated the value of the MiG-29K fighter deal at around $1.5bn.
Russia supplies 70 per cent of India's military hardware, but in recent years New Delhi has looked to other military suppliers, including Israel and the US.
Together with Brazil and China, Russia and India make up the so-called BRIC grouping of major developing economies seeking to promote a multipolar world economy not dominated by the US.
But at just over $7.5bn in 2009, trade turnover remains minuscule and the two countries will aim to increase it to $20bn by 2015.
According to Indian officials, energy is emerging as a new focus of co-operation between oil and gas-rich Russia and energy-starved India, which is always on the lookout for new fuel sources to power its growing economy.
The strong ties between Moscow and New Delhi date back to the 1950s.
But India has in recent years also taken care to balance this friendship by fostering closer relations with the US.