Russia has strengthened its anti-aircraft defences in Syria by moving a cruiser towards the coast and deploying new missiles at its main base.
The Moskva cruiser's long-range air defence system will provide cover for Russian aircraft, as will the S-400 missiles which arrived on Thursday.
The row over Turkey's downing of a Russian combat jet on Tuesday rages on.
Turkish President Recep Tayyip Erdogan has warned Russia not to "play with fire" with its operations in Syria.
Russia has suspended its visa-free arrangement with Turkey and is planning to introduce a wide range of economic sanctions. President Vladimir Putin wants an apology from Turkey before he will speak to Mr Erdogan.
Moscow rejected Turkey's assertion that its SU-24 fighter jet had flown into Turkish air space for 17 seconds after ignoring repeated warnings to change course.
A senior Russian commander said the jet had in fact been "ambushed" by two Turkish F-16s patrolling the area for more than an hour beforehand.
He said Russian and Syrian radar data proved that a Turkish jet had fired a missile from 2km (1.2 miles) inside Syria. Turkey did not respond to the allegation. Read more at The BBC