ISTANBUL — The Turkish Army suffered mass casualties in an airstrike in northwest Syria late Thursday, an attack that could dramatically change the course of the Syrian war as fears grow of a direct conflict between Russia and Turkey, a NATO member.

At least 33 Turkish soldiers were killed and more than 30 wounded, said Rahmi Dogan, the Turkish governor of the southern province of Hatay, where the Turkish casualties were arriving.

Turkish officials said the strike had been carried out by Syrian government forces, but Russian jets have been conducting most of the airstrikes in the area in recent weeks. Turkish protesters in Istanbul converged on the Russian Consulate there early Friday, chanting “Murderer Russia! Murderer Putin!”

Turkish officials have avoided blaming the Russian government for aggression against their forces in Syria, hoping to avoid a direct confrontation with Russia’s much stronger military and to keep a line open for talks with Russia’s president, Vladimir V. Putin.