Two Russian-built reactors have been in commercial operation in Tamil Nadu's Kudankulam. (File)

India and Russia today signed a pact to build six more nuclear reactors at a new site in India following summit talks between Russian President Vladimir Putin and Prime Minister Narendra Modi in New Delhi.

The two leaders also agreed to cooperate on India's plan for a manned space mission.

Russian state-owned reactor manufacturer Rosatom said in a statement that the two countries want to build six Russian-design nuclear reactors on a new site in India, boost nuclear cooperation in third countries and new nuclear technologies and are considering building nuclear plants together.

The firm said Russia would offer to build its third-generation VVER reactor on the new site and would increase the level of participation of Indian companies in the project.

A Rosatom official told Reuters the pact is not a firm contract yet, but an agreement to work towards a contract.

India has not chosen the new site yet, which could be controversial as there have been vehement protests against new nuclear sites.

If confirmed, the agreement would be one of the biggest nuclear industry deals in recent years, and would bind the two countries for decades.

Two Russian-built VVER-1000 reactors have been in commercial operation in Tamil Nadu's Kudankulam since 2014 and 2017 respectively. Construction on two more started last year with a target for commercial start-up in 2025 and 2026.

Last year, the Russian and Indian governments signed an agreement to build reactors 5 and 6 on the site and Putin said at the time that Russia is ready to build a dozen reactors in India over the next 20 years.

"We expect to start building a series of new units at a second site in India in the near future," Rosatom Director-General Alexey Likhachev said in a statement.

Rosatom has become the world's largest nuclear reactor builder as the financial problems of the two big Western firms Westinghouse Areva have crimped their ability to develop nuclear plants abroad.

Rosatom operates 35 reactors in Russia with a combined capacity of 28 gigawatt and says it has a portfolio of 36 nuclear power plant projects in 12 countries.

Westinghouse and Areva, now owned by EDF, have for years negotiated deals to build reactors in India but have made little progress, partly because Indian nuclear liability legislation gives reactor manufacturers less protection against claims for damages in case of accidents.

© Thomson Reuters 2018