NATO's secretary-general has said a "substantial" build-up of Russian forces in Syria includes ground troops.
Although Vladimir Putin has said he would not put troops in Syria, Russian military chiefs have raised the prospect of "volunteer" soldiers joining the fighting.
If employed, the tactic would have echoes of the crisis in Ukraine, where Russian troops apparently helped rebels while Moscow denied its soldiers were involved in the conflict.
Jens Stoltenberg also said he doubted Russia's claim that violations of Turkey's airspace were a mistake because there were two incursions and they lasted longer than a few seconds.
He said the incidents were "very serious" and added: "It doesn't look like an accident, and we've seen two of them over the weekend".
The Russian government is reportedly looking into the second alleged violation of Turkish airspace, after the Kremlin's ambassador was once again summoned in Ankara for an explanation.
Moscow has said the Syrian airbase from which Russian planes were flying missions, Khmeimim, was about 19 miles (30km) from the Turkish border and that its aircraft had to approach it from the north in certain weather conditions.
As Mr Stoltenberg was speaking, Russian media quoted a senior politician as saying Moscow would consider extending airstrikes against militants to Iraq if a request came from Baghdad.
Valentina Matviyenko, head of Russia's upper chamber of parliament, said Russia had not received such a request from the Iraqi government.
Footage from strikes this week has made it onto Youtube:
