(CNN) At least 33 Turkish soldiers were killed in Syria's Idlib province on Thursday in an aerial attack by Russian-backed Syrian regime forces, Turkish officials said, raising the specter of full-blown conflict.

Thirty-five soldiers were injured in the attack and have been evacuated to hospitals in Turkey, according to officials.

The attack comes days before a Turkish deadline on Syrian government troops to withdraw from areas recaptured in recent months from rebels in northwest Syria.

Turkey, which backs some Syrian anti-government fighters, condemned the Syrian attack as a "nefarious attack against heroic soldiers in Idlib who were there to ensure our national security."

Turkish Defense Minister Hulusi Akar said the attack happened "despite coordination with the officials of the Russian Federation who were on the ground when this attack happened." Akar said ambulances were hit during the strikes.

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