The S-400 missile systems are expected to transform the strategic balance in the subcontinent. India wants to use the long-range missile systems to tighten its air defence mechanism, particularly along the nearly 4,000-km-long India-China border.

There have been indications by New Delhi that it will go ahead with the S-400 deal notwithstanding the US sanctions. "As and when the government approves it, delivery (of the missiles) will be in 24 months," Air Chief Marshal BS Dhanoa has told reporters.

Under a new law - Countering America's Adversaries through Sanctions Act or CAATSA - the US imposes sanctions on any country that has "significant transactions" with Iran, North Korea or Russia.

The law primarily deals with sanctions on Russian interests -- such as its oil and gas industry, defence and security sector, and financial institutions -- in the backdrop of Russia's military intervention in Ukraine and alleged meddling in the US Presidential elections in 2016.

Vladimir Putin and Prime Minister Narendra Modi will also discuss key regional and global issues, including US sanctions on import of Iranian crude oil. Mr Putin will also meet President Ram Nath Kovind tomorrow.

India is also planning to acquire 4 Krivak-class frigates from Russia, two of which will be built in India. Under the $2.5 billion deal, two ships will be built at the Goa shipyard and two others will be acquired from Russia. India already operates six similar frigates.

Known as Russia's most advanced long-range surface-to-air missile defence system, the S-400 is manufactured by Almaz-Antey and has been in service since 2007. The missiles, which include launchers, radars and command centre, can hit enemy missiles and aircraft 400 km away. The S-400 radars can detect targets 600 km away.

China was the first foreign buyer to seal a government-to-government deal with Russia in 2014 to procure the lethal missile system and Moscow has already started delivery of an undisclosed number of the S-400 missile systems to Beijing.

PM Modi and Mr Putin will also hold the 19th India-Russia summit tomorrow during which around 20 agreements are expected to be signed. Russia is one of only two countries with which India holds annual bilateral summits, the other being Japan.