NEW DELHI: Despite the looming threat of US financial sanctions, India on Friday inked $5.43 billion (Rs 40,000 crore) deal for five advanced S-400 Triumf

squadrons with Russia. The S-400 deal has been at the centre of the ongoing visit of

's President Vladimir Putin to India.

The $5.43 billion (over Rs 40,000 crore) deal is among India's largest defence deals ever made. Here is all you need to know about the anticipated S-400 defence deal between India and Russia.

What is S-400

It is an air defence missile system that can take down enemies' aircraft in the sky from the surface itself.

The S-400 is known as Russia's most advanced long-range surface-to-air missile defence system, capable of destroying destroying hostile strategic bombers, jets, missiles and drones at a range of 380-km.

The S-400 is an upgraded version of the S-300 systems. The missile system, manufactured by

, has been in service in Russia since 2007.

Why does India want S-400

In the words of Air Force Chief BS Dhanoa, S-400 would be like a "booster shot" in the Indian Air Force's (IAF) arm.

India needs to be well-equipped against neighboring threats. Pakistan has over 20 fighter squadrons, with upgraded F-16s, and inducting J-17 from China in large numbers. China has 1,700 fighters, including 800 4-Gen fighters.

Whereas, a shortfall of over fighter squadrons has severely affected IAF's efforts to pose a challenge to the enemies.

“No country is facing the kind of grave threat that India is confronted with. Intentions of our adversaries can change overnight. We need to match force level of our adversaries,” IAF Chief Dhanoa said at a press conference this week.

Strategic autonomy

An increasingly-assertive US has been punishing countries doing 'significant' deals with Iran or Russia with sanctions. Going ahead with the 'significant' defence deals will also be a message about India's 'strategic autonomy' despite its dependence on both US and Russia.

What is stopping India from making the deal

The threat that the United States of America might exercise sanctions on India for trading with Russia.

The US administration is required under a domestic law, Countering America's Adversaries through Sanctions Act or CAATSA to impose sanctions on any country that has “significant transactions” with Iran, North Korea or Russia.

The Act 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 its military intervention in Ukraine and its alleged meddling in the 2016 US Presidential elections.

In September, a senior administration official told reporters that purchase of major military system like multi-billion S-400 missile defense system would be considered as a "significant transaction and thus has a potential" for CAATSA sanctions.

Which countries have it ?

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.

Last year Moscow signed a contract to sell the systems to Turkey, raising criticism from Ankara's NATO allies, notably Washington, which has threatened to block delivery of its F-35 stealth aircraft to Turkey.

Other than India, Russia is also negotiating with Qatar to sell its S-400 Triumf.

(With inputs from agencies)