The S-400 Triumf anti-aircraft weapon system can engage all types of aerial targets including aircraft, drones and ballistic and cruise missiles within a range of up to 400 kilometers (250 miles), at an altitude of nearly 30 kilometers (19 miles).
The system is fitted with radar that is capable of tracking up to 300 targets within a range of more than 595 kilometers (370 miles).
The S-400 entered service in 2007 and since then, its units have been deployed around Moscow and elsewhere in Russia.
The system being put on combat alert in Crimea came after a number of incidents involving the United States and Ukraine last year.
During the Caucasus-2016 drills on September 8, three US reconnaissance aircraft approached Crimea's southern border. Also, the US reconnaissance plane RC-135 took off from a Greek airbase and flew 70 kilometers, to the Russian city of Sevastopol.
On January 13, 2016 Ukraine's air defense units held exercises in the Kherson region which borders Crimea, something that prompted Russia to put Crimean air defense forces on full combat alert.
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