India will ask the United States for a special waiver for the purchase -- as discussed by their defence and foreign ministers in New Delhi last month -- but Washington has signalled there is no guarantee it will accede.

The US is in a difficult position as it seeks to contain China through better ties with India, which like Washington has been unnerved by Beijing's growing assertiveness.

Washington and New Delhi announced plans last month for joint military drills in 2019, and agreed on the exchange of sensitive military information.

But Russia remains India's main arms supplier, and a string of new deals with the Asian giant would be a major win for Moscow -- and a big snub to the US.

Putin and Modi, who appear to enjoy a personal rapport, are also likely to discuss a deal for four Krivak-class frigates worth $2 billion and 200 light utility Ka-226 helicopters pegged at $1 billion.

India and Russia have already concluded an Inter-Governmental Agreement for the choppers, 60 of which would be built in Russia and the rest in India.

"Russia is a time-tested friend. I am really glad some spine has finally been shown by India," R.R. Subramanian, a Delhi-based strategic affairs analyst, told AFP.

"It's about time we... showed that we are not going to be pushed around by Washington."

Experts say India needs the sophisticated S-400 system to fill critical gaps in its defence capabilities, in view of China's rise and perceived threats from Pakistan, against whom India has fought three wars.

Indian Air Force Chief Birender Singh Dhanoa said on Wednesday that the S-400 and the 36 Rafale fighter jets purchased from France -- a 2016 deal mired in political controversy -- represent a "booster dose" for the country.

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