BEIRUT, Lebanon — A Russian airstrike killed three Turkish soldiers in northern Syria on Thursday, an attack that both countries described as an accident. Still, it injected new tension into a volatile corner of the Syrian battlefield as new clashes erupted between Turkish-backed rebels and Russian-backed government forces.

Russia’s president, Vladimir V. Putin, apologized in a phone call to his Turkish counterpart, Recep Tayyip Erdogan, calling the deaths a “tragic incident,” according to official accounts from their governments. One Turkish official called it “friendly fire.”

Both countries appeared eager to avoid derailing their new cooperation in Syria, where they have been seeking to lead a new effort to resolve the complex six-year-old conflict, even as they support combatants on opposite sides.

But the incident between Turkey, a NATO member, and Russia, ratcheted up an already risky situation in northern Syria. It came as Syrian rebels fighting alongside Turkish troops clashed in that same area for the first time with pro-government forces working with Russian air cover.