New Delhi - Russia will build at least 12 nuclear reactors in India by 2035 and agreed to manufacture advanced helicopters as both countries on Thursday signed 20 pacts in oil, gas, defence, investment and other key sectors.

Indian Prime Minister Narendra Modi and Russian President Vladimir Putin, in their annual summit, vowed to take the close ties to a new level as both sides outlined a new vision for nuclear energy cooperation.

"We have outlined an ambitious vision for nuclear energy," Modi said at a joint press briefing. "It will have the highest standards of safety in the world. It will also include manufacture of equipment and components in India," he said.

Russia has supplied two reactors to a plant at Kudankulam in southern India under a long-delayed agreement, and has been pushing to supply many more after deals were signed in 2010 and 2008.

Putin's visit was his first to India since Modi was elected in May, and comes as sanctions-hit Moscow seeks to strengthen energy, defence and strategic ties in Asia.

The president is seeking new markets for Russia's natural resources as its economy reels under US and EU sanctions over its backing of an uprising in Ukraine and annexation of Crimea.

The two countries on Thursday signed 16 agreements including a deal for Russia to manufacture in India one of its most advanced helicopters and a proposal to set up factories to make Russian spare parts for military equipment.

Russia has traditionally supplied 70 percent of India's military hardware.

India, whose domestic industry struggles to manufacture high-tech arms, is in the middle of a defence spending binge.

Total bilateral trade stands at just $10 billion a year despite strong Russia-India ties that date back to the 1950s after the death of Stalin.

Calling Russia a "pillar of strength" for India, Modi said the strategic partner will remain New Delhi's "most important defence partner" though options have increased.

"President Putin and I discussed a broad range of new defence projects. We also discussed how to align our defence relations to India's own priorities, including Make in India.

"I am pleased that Russia has offered to fully manufacture in India one of its most advanced helicopters. It includes the possibility of exports from India. It can be used for both military and civilian use. We will follow up on this quickly," Modi said after around three-and-half-hour long meeting with Putin.

The strategic vision document on nuclear cooperation said the two sides have decided to fast-track the implementation of agreed cooperation projects for nuclear power plants.

"Both sides will strive to complete the construction and commissioning of not less than 12 units in the next two decades, in accordance with the Agreement of 2008.”

The 20 agreements signed between the two countries provided for cooperation in a spectrum of areas including nuclear energy, oil and gas, health, investment, mining, media and wind power.

Terming the talks "substantive", Putin said Russia will support "joint high technology" projects and will facilitate the establishment of an Indian operator of mobile communications besides offering India its civil aviation aircraft.

Putin said Russia and India have been long cooperating in the defence sector and the two countries have reached a level of joint production of military hardware.

Indicating that Russia was not averse to production of military hardware in India, he said both the countries have already successfully implemented the Brahmos missile programme and was working on a jet fighter and the multi-role transport aircraft programme.

During the talks, the Indian side agreed to expeditiously identify a second site, in addition to Kudankulam, for the construction of the Russian-designed nuclear power units in India.

On defence cooperation, Modi said a broad range of new defence projects were discussed that covered how to align "our defence relations to India's own priorities, including Make in India."

Noting that energy security was critical to India's economic development, Modi said both countries will set an ambitious agenda for partnership in oil and natural gas.

The pacts signed included an agreement which provides for cooperation, covering joint exploration and production of hydrocarbons, long term LNG supplies and joint study of a hydrocarbon pipeline system connecting Russia with India.

Moscow recently sealed a military cooperation pact with Pakistan after lifting its embargo on arms supplies to India's arch-rival, and is in talks to supply Islamabad with combat helicopters.

Putin is also meeting with Indian President Pranab Mukherjee and attending the World Diamond Conference, with both sides keen on ramping up direct exports to India.

Russia is the world's top producer of rough diamonds and the majority of them pass through India, where a cheap workforce cuts and polishes the gemstones before most are exported again for use in jewellery.

But only about a fifth of rough produce is sold directly from Russian mines to India, with the rest passing through diamond hubs such as Antwerp and Dubai.