Russian air strikes killed three Turkish soldiers and wounded 11 others during an operation against Islamic State in northern Syria on Thursday, the Turkish military said.
The Kremlin said Russian President Vladimir Putin had called Turkish counterpart Tayyip Erdogan and expressed his condolences, blaming the misdirected strikes on poor coordination between Moscow and Ankara.
The Kremlin has issued a statement (in Russian) saying President Putin had, in a telephone call with his Turkish counterpart, "expressed condolences over a tragic incident which resulted in the deaths of several Turkish troops in the al-Bab area".
The incident highlighted the risk of unintended clashes between the numerous outside powers involved in Syria's complex six-year-old war. Besides Russia and Turkey, they include Lebanon's Hezbollah and other Iranian-backed militias, and members of a U.S.-led coalition fighting Islamic State.
The incident comes after a warming of relations between the two countries, which had previously been strained after Turkey downed a Russian jet near the Syrian border two years ago.
At the end of 2015, Russia and Turkey risked military confrontation as the Turkish military shot down a Russian jet which it said had violated its airspace. By the end of 2016, the two, who back opposite sides in the Syrian war, were co-operating to reach a ceasefire in Aleppo.
Russia is a key ally of Syrian President Bashar al-Assad, while Turkey supports the rebels opposing him. But despite supporting opposing sides, Russia and Turkey have joined forces in recent months to try to end the Syria conflict.They carried out their first joint air strikes on IS targets in the al-Bab area in January, just weeks after securing a ceasefire deal between rebels and government forces that has held in many parts of Syria since the end of last year.
The Dogan news agency says 66 Turkish troops have been killed in Syria since Turkey began its operations there last August to drive out IS, as well as stop the advance of the Kurdish YPG militia - which Turkey considers a terror organisation.