President Vladimir Putin has ordered the Russian military to take “extremely tough” action against any threat to its forces in Syria.
Such targets must be “immediately destroyed”, he told defence officials in televised comments.
Mr Putin did not elaborate on specific threats but Turkey and Russia are currently locked in a spat over the downing of a Russian jet by Ankara.
Turkey said the bomber had entered its airspace.
But Russia has insisted the plane did not cross the border and that it gave advance notice of the flight path to the US.
A Russian pilot was killed in the downing of the Su-24 bomber, as was a marine sent on a rescuemission, badly damaging the normally close ties between Ankara and Moscow.
Russia began air strikes in Syria in September, at what they said was a request from President Bashar al-Assad.
Mr Putin told military leaders “terrorists” in Syria pose a direct threat to Russia.
He said a Russian military base in the country had been reinforced with new air units and defence equipment.