Washington has been persistent in its attempts to pressure its NATO ally Turkey to abandon a similar deal for the procurement of Russian air defence systems. The attempts, however, have not borne fruit, despite US threats to freeze the supplies of its top jet, the F-35, to Ankara.

Washington has been trying to convince India to ditch a deal with Moscow on the purchase of S-400 air defence systems by offering New Delhi alternatives to the system, Randall Schriver, the assistant defence secretary for Indo-Pacific Security Affairs told Congress during a hearing.

"We are very keen to see them [India] make an alternative choice. We're working with them to provide potential alternatives [to the S-400s]", he said.

At the same time, he noted that potential US sanctions against India if it proceeds with the deal would make it "an unfortunate decision" by New Delhi — with which Washington sees "an emerging partnership". He added that despite this, the US will proceed with punitive measures under CAATSA legislation, which is designed "to impose cost on Russia".

The Countering America's Adversaries Through Sanctions Act (CAATSA) is a US law that gives Washington the right to impose economic sanctions on any state if it decides to obtain Russian military equipment, such as S-400 air defence systems. Russia has slammed the bill as a declaration of "an all-out trade war" and accused Washington of "Russophobic hysteria".

READ MORE: US, India to 'Sort Out' Sanctions Waiver for S-400 Purchase — Mattis

India is not the first country that the US has tried to convince to abandon the purchase of Russian S-400s. Washington has been trying for over a year to achieve the same goal with Turkey, threatening to suspend F-35 deliveries, despite the fact that Ankara has already paid for the top jets. The Turkish president slammed the US threats and reaffirmed the country's stance on sticking to its deal with Russia.