Russian air strikes accidentally killed three Turkish soldiers and wounded 11 others during an operation against Islamic State in northern Syria, the Turkish military says.
"During an operation by a Russia Federation warplane against Islamic State targets in the region of the Euphrates Shield operation in Syria, a bomb accidentally hit a building used by Turkish Army units," the military said in statement on Thursday (local time).
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 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 US-led coalition fighting Islamic State.
Russia is a key ally of Syrian President Bashar al-Assad, while Turkey supports the rebels opposing him. In 2015, Turkey shot down a Russian air force jet that it said had crossed into Turkish airspace, though Moscow denied that happened.
The two countries have since repaired relations, and Thursday's Kremlin statement said the two leaders had agreed to step up military coordination against Islamic State.
Turkish-backed Syrian rebels meanwhile pursued a major offensive against the IS-held Syrian city of al-Bab, 30 km south of the border with Turkey.
A rebel commander said fighters of the Free Syrian Army (FSA), working with Turkish commanders, were moving forward from territory near the western gates of the city, which they had stormed on Wednesday.
The capture of the town would deepen Turkish influence in an area of northern Syria where it has created a de facto buffer zone. It launched its Euphrates Shield operations in August, backing Syrian rebels with special forces, tanks and aircraft to sweep Islamic State from its border area and stop the advance of a Kurdish militia.
Al-Bab is a major economic hub for the militants and lies on a key crossroads for the region north of Aleppo.
The Turkish military said on Thursday it had killed 44 militants in aerial and artillery strikes and clashes in northern Syria. Five Turkish soldiers were killed in the clashes, the private Turkish news agency Dogan said.