Turkish media are reporting that Turkish and Russian officials have signed a financing agreement for the purchase of a Russian-made anti-missile system worth $2.5 billion.

The Hurriyet Daily News newspaper said the loan deal for four S-400 surface-to-air missile batteries was signed in the Turkish capital Ankara on Friday.

Turkish President Recep Tayyip Erdogan announced in September that Turkey had signed a deal to buy Russia's most advanced air defense system, drawing concerns from some of Turkey's NATO allies.

Some NATO countries have expressed worries that the S-400 system is not compatible with the alliance's weapons systems.

Turkey plans to finance part of the purchase through a Russian loan. Erdogan told a group of journalists that the loan would be set in Russian rubles.