Here are 10 key details about the S-400 Triumf missile system:
It can hit aircraft, cruise missiles and even fast-moving intermediate range ballistic missiles. The S-400 has three components -- the missile launchers themselves, a powerful radar and a command centre.
The radar can track targets 600 km away. For example, S-400 systems deployed in India can track aircraft flying well outside the country's borders, which is a major threat deterrence to the enemy's air force.
Manufactured by the Russian government-owned arms company Almaz-Antey, the S-400 Triumf is known as the SA-21 Growler by the North Atlantic Treaty Organisation or NATO.
NATO members consider the S-400 a major threat because of its sheer long range. The S-400 can spoil an enemy's attempt at air dominance, and in turn help stop the advance of ground troops.
Analysts say the Russian missile system can drastically change the tide of a military campaign by denying air cover over a vast area. For example, Russia has deployed the S-400 in Syria to protect its military assets. With a range of 400 km, the missile can engage warplanes flying well outside the Syrian borders.
The S-400 can engage almost every type of modern warplanes. Advanced jet fighters with stealth capabilities will also find it difficult to operate in a battlespace where the S-400 is deployed.
The first S-400 systems became operational in 2007 and entered service with the First Air Defence Corps responsible for defending Moscow.
The launchers contain the 48N6 missile series, which uses a high-explosive fragmentation warhead.
The S-400 is said to be comparable to the US Patriot anti-ballistic and anti-aircraft missile system.
China has also bought an unspecified number of S-400 missile systems. It is not known which variant of the missile -- the 400 km range or a lower range -- China has deployed.