By Jonathan Marcus, BBC, 01 December 2015
The shooting down of a Russian Su-24 warplane by Turkish fighters has prompted Moscow to reinforce its contingent in Syria.
Russian warplanes have now been seen carrying air-to-air missiles and a Russian warship - the Moskva - was moved closer to the Syrian coast, to enable its air defence missiles to provide additional cover over the Russian air base near Latakia.
But the most significant reinforcement has been the deployment of a sophisticated S-400 air defence system. Its large tubular launchers and associated radar and command vehicles are now in position at the Russian base.
The S-400 is one of Russia's most modern air defence systems with an extraordinary reach. From the airbase near Latakia the S-400's surface-to-air missiles could strike targets in an arc that takes in much of Israel; the eastern Mediterranean (including Cyprus where British jets are based); and northwards to cover a large part of Turkey beyond the Syrian border.