President Recep Tayyip Erdogan said a deposit had already been paid. The deal is thought to be worth $2.5bn (£1.9bn).Turkey has the second-largest army in Nato. The alliance reacted sceptically to the decision, saying the system was not compatible with its equipment.
Turkey has been establishing closer links with Russia after its recent souring of ties with the US and Europe.Mr Erdogan's government objects to US military support for the YPG Syrian Kurdish rebels, who are linked to rebel Kurds in Turkey.Russia says the S-400 system has a range of 400km (248 miles) and can shoot down up to 80 targets simultaneously, aiming two missiles at each one.
Russia deployed the S-400 at its air force base near Latakia in Syria in December 2015, after Turkish jets had shot down a Russian Su-24 warplane on the Syria-Turkey border.That incident caused a diplomatic rift between Russia and Turkey, but President Erdogan later patched up his quarrel with President Vladimir Putin.
A military adviser to Mr Putin, Vladimir Kozhin, said the S-400 contract with Turkey was "strictly compatible with our strategic interests". "On that score, one can quite understand the reaction of some Western countries who are trying to put pressure on Turkey."Mr Erdogan, quoted by Turkey's Hurriyet daily, voiced displeasure with unnamed Western partners who were "seeking enormous amounts of money" for military drones.
He said Turkey had killed 90 YPG "terrorists" in the past week with Turkish drones - developed because the Western ones were too expensive."We are responsible for taking security measures for the defence of our country," he stressed.
The missile deal is clearly a rebuff to Nato after the US and Germany withdrew Patriot air defence batteries from Turkey.In 2015, Turkey urged its Nato allies to keep those batteries positioned on the Turkish-Syrian border. Speaking to AFP news agency, an unnamed Nato official said: "No Nato ally currently operates the S-400". They added: "Nato has not been informed about the details of any purchase."
Germany's Foreign Minister, Sigmar Gabriel, said Berlin would put all arms exports to Turkey on hold due to the deteriorating relationship between the two nations.Mr Gabriel's counterpart in Ankara, Mevlut Cavusoglu, said the comments were inappropriate for a foreign minister.Relations between the two countries have deteriorated since Turkey arrested a Turkish-German journalist in February as part of a crackdown on political opponents in the country.
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