India announced the multi-billion-dollar purchase of Russia's S-400 air defence system and several other deals with Moscow Friday during a visit to New Delhi by President Vladimir Putin.

The agreements, announced following Putin´s talks with Indian Prime Minister Narendra Modi, come despite threats of sanctions by the United States on countries buying Russian military hardware.

"The sides welcomed the conclusion of the contract for the supply of the S-400 long-range surface-to-air missile system to India," a joint Indian-Russian statement said.

Other agreements — following a meeting between Putin and Prime Modi that began with a hug — were in railways and fertilisers and space, with Russia undertaking to train astronauts for India´s first crewed space mission in 2022.

India's decision to purchase the S-400 flies in the face of the United States imposing sanctions on countries buying Russian military hardware, as happened with China last month.

US-Russia ties are plumbing depths not seen since the Cold War, plunging further still this week as Western powers accused Moscow of orchestrating a brazen string of global cyber attacks.

However, Washington is in a tricky position with India, seeking to bolster ties with the Asian giant to counter China's growing assertiveness, something which has also rattled New Delhi.

Last year India and China had a military standoff over a Himalayan plateau claimed by Beijing and Bhutan, a close ally of India.

China has also perturbed India by loaning large amounts of money to Indian Ocean countries where it has long held sway.

"It is also important for India to keep friendly relations with Russians so that they don't completely move into the Chinese camp," said Manoj Joshi from New Delhi-based Observer Research Foundation think-tank.

"Also, the fact is that unlike the US, Russian regional interests largely overlap with India's regional policy," he told AFP.

Washington and New Delhi announced plans last month for joint military drills in 2019, and agreed on the exchange of sensitive military information. The US is now India´s second-biggest arms supplier.

India therefore could expect to gain a sympathetic ear if it seeks a waiver from the US sanctions, but President Donald Trump's administration has signalled this is unlikely.