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 (local time).
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 suspended its visa-free arrangement with Turkey and was 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.
Russian and Syrian radar data proved that a Turkish jet had fired a missile from two kilometres inside Syria, he said. Turkey did not respond to the allegation.
The long-range S-400 system was delivered to Russia's Hmeimim air base just 50km from the border with Turkey.
Russian aircraft have been using the base to attack opponents of Syrian President Bashar al-Assad since September 30.
Turkey, which is a member of Nato and of a US-led coalition in the region, insists Mr Assad must step down before any political solution to the Syrian conflict is found.
Russia 'playing with fire', Turkey warns
In a televised speech, Mr Erdogan warned Russia it was "playing with fire to attack the Syrian opposition, who have international legitimacy, under the pretext of fighting against Daesh [another name for IS]".
He accused Moscow of seeking to use the downing of the jet as an "excuse to make unacceptable accusations against us", and accused Russians of "mistreating" Turkish citizens who were in the country for a trade fair.
He hoped to meet Mr Putin face to face on the sidelines of the climate summit in Paris next week "to bring the issue to a reasonable point. We are disturbed that the issue has been escalated".
While he has refused to apologise, Mr Erdogan did say that had Turkey known the plane was Russian, "maybe we would have warned it differently".
But Mr Putin has firmly rejected any suggestion Turkey did not recognise the plane as Russian. It was easily identifiable and its co-ordinates had been passed on to Turkey's ally, the US, he said.
President Putin's foreign affairs adviser Yuri Ushakov said the Kremlin had received Mr Erdogan's request for a meeting but he would not say whether such a meeting was possible.
"We have seen that the Turkish side hasn't been ready to offer an elementary apology over the plane incident."
Announcing the suspension of a visa-free travel regime with Turkey from January 1, Russian Foreign Minister Sergei Lavrov said he believed the Turkish leadership had "crossed the line of what is acceptable".
Russia said it was drafting a wide-ranging list of economic sanctions against Turkey that would hit food imports and joint investment projects among other things.
Turkey and Russia have important economic links. Russia is Turkey's second-largest trading partner, while more than three million Russian tourists visited Turkey last year.