S-400
© Sergey Karpukhin / ReutersRussian S-400 Triumph medium-range and long-range surface-to-air missile systems during the Victory Day parade at Red Square in Moscow, May 9, 2015
Indian Prime Minister Narendra Modi arrived in Moscow on Wednesday following the approval of the biggest arms deal with Russia since 2001.

India's Defense Acquisition Council (DAC) last week agreed the purchase of five S-400 air defense systems from Russia.

The S-400 systems are among the 'crown jewels' of Russia's defense capability, Jon Grevatt Asia-Pacific defense industry analyst for IHS Jane's, told Bloomberg.

"Russia and India have a very strong partnership that the US can only aspire to," said Grevatt. "Sales from America may ebb and flow, but the sales from Russia will remain strong because there are so many ongoing programs between the two countries."

The cost of the deal was not disclosed but according to estimates the air defense systems could cost India $4.5-$6 billion. It will become the biggest defense deal between the two countries since 2001 when India agreed to buy 140 Sukhoi Su-30MK fighter jets.

During his two-day visit to Moscow PM Modi will meet President Putin. The pair will host the Russia-India summit in Moscow on Thursday where energy cooperation, investment and trade are to be discussed.

Moscow and New Delhi have targeted a five-fold increase in mutual investment as well as a significant boost to trade during the next ten years.

India's Prime Minister hopes the "first state visit to Russia would bring bilateral relations to a new level."

S-400 is the latest upgrade of its renowned predecessor, S-300 air defense system. It can hit multiple-range air targets, including fighter jets, bombers, drones, ballistic and cruise missiles at ranges of about 400 kilometers. The S-400's digital target acquisition system and radars allow detecting and hitting several different targets at one go, which makes the system a game-changer in modern warfare.