Turkey has signed a controversial deal with Russia to arm its forces with Russian S-400 anti-aircraft missiles.

President Recep Tayyip Erdogan said a deposit had already been paid. The deal is thought to be worth 2.5 billion U.S. dollars.

Russia says the S-400 system has a range of 400km and can shoot down up to 80 targets simultaneously, aiming two missiles at each one.

Turkey has the second-largest army in NATO.  According to BBC, the alliance reacted skeptically 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.

A military adviser to President 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.”

BBC says the missile deal is clearly a rebuff to NATO, after the US and Germany withdrew Patriot air defense 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.”

The New York Times says the deal comes as relations between Russia and the West are at a particularly low point. 

Although a prospective missile purchase from Russia was made public several months ago, Mr. Erdogan’s announcement was the first confirmation that Turkey had transferred money to pay for the missile system, known as the S-400.