BBC News:
Turkey has signed a controversial deal with Russia to arm its forces with Russian S-400 anti-aircraft missiles. President Recep Tayyip Erdogan said Ankara had already paid the deposit on the deal, estimated to be worth $2.5bn (£1.9bn).
Turkey has the second-largest army in Nato. Alliance members are urged to buy compatible weapons systems. Turkey has been establishing closer links with Russia after its recent souring of ties with the US. Turkey 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.
