Just four days after announcing their planned delivery, Russian officials announced Friday that Russia has already started delivery of S-300 air defense systems to Syria, and that Syria is in possession of some such systems.

Exactly how many have been delivered so far is unclear. Russia has said that as many as eight systems could be delivered, with the goal of providing total coverage for all Syrian airspace from start to finish.

The S-300 is a substantial upgrade from Syria's existing air defense system, the S-200. The new system has advanced radar covering broad swathes of territory, and missiles capable of intercepting even advanced aircraft.