India agreed a deal with Russia to buy S-400 surface to air missile systems on Friday, the Kremlin said, as New Delhi disregarded US warnings that such a purchase could trigger sanctions under US law.
Although there was no public signing, the deal was sealed during President Vladimir Putin's ongoing visit to New Delhi for an annual summit.
"The deal was signed on the fringes of the summit, Kremlin spokesman Dmitry Peskov told Reuters. The contract is estimated to be worth more than $5 billion and gives the Indian military the ability to shoot down aircraft and missiles at unprecedented ranges.
But the United States has said countries trading with Russia's defense and intelligence sectors would face automatic sanctions under a sweeping legislation called Countering Americas Adversaries Through Sanctions Act (CAATSA).
A State Department spokesperson said this week that the implementation of the sanctions act would be focused at countries acquiring weapons such as the S-400 missile batteries.
Last month, the United States imposed sanctions on China's military for its purchase of combat fighters as well as the S-400 missile system it bought from Russia this year.
India is hoping that President Donald Trump's administration will give it a waiver on the weapons systems which New Delhi sees as a deterrent against China's bigger and superior military.
After summit talks between Putin and Modi, the two countries signed eight agreements covering space, nuclear energy and railways at a televised news conference.