India and Russia signed the $5 billion S-400 air defence system deal on Friday, 5 October after wide-ranging talks between Prime Minister Narendra Modi and Russian President Vladimir Putin, official sources said.
The agreement was signed in the presence of both the leaders at the 19th India-Russia annual bilateral summit.
The S-400 missile defence system deal could result in US sanctions under the Countering America’s Adversaries Through Sanctions Act (CAATSA) instituted by the US Congress on arms purchases from Russia.
The US has urged its allies to forgo transactions with Russia, warning that the S-400 missile defence system that India wanted to buy would be a "focus area" for it to implement punitive sanctions against a nation undertaking "significant" business deals with the Russians.
American lawmakers, however, have allowed the possibility of a presidential waiver.
India wants the long-range missile systems to tighten its air defence mechanism, particularly along the nearly 4,000-km-long Sino-India border.
S-400 is known as Russia's most advanced long-range surface-to-air missile defence system. China was the first foreign buyer to seal a government-to-government deal with Russia in 2014 to procure the lethal missile system. Moscow has already started delivery of an undisclosed number of the S-400 missile systems to Beijing.
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