Turkey says a Russian airstrike has accidentally killed three Turkish soldiers in northern Syria.
A military statement said the Russian military was carrying out an operation targeting Islamic State at the time of the incident.
It said Russian President Vladimir Putin has called his Turkish counterpart Recep Tayyip Erdogan to offer his condolences.
The Kremlin said the both sides have agreed to step up their military coordination in Syria.
Russia and Turkey had been on opposing sides of the nearly six-year Syrian conflict, with Moscow backing Syrian President Bashar al-Assad while Ankara supported the rebels.
But the two countries have worked together in recent months and helped broker Syria peace talks last month in Kazakhstan.
Russia and Turkey, along with Iran, agreed on a plan to monitor a partial Syrian cease-fire and support efforts to find a political solution to the conflict.