India agreed a deal with Russia to buy S-400 surface to air missile systems on Friday, the Kremlin said, as New Delhi disregarded U.S. warnings that such a purchase could trigger sanctions under U.S. 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 America’s Adversaries Through Sanctions Act (CAATSA).

