President Vladimir Putin has sent condolences after a Russian air strike accidentally killed three Turkish soldiers in northern Syria.
The soldiers were supporting Syrian rebels in efforts to capture the city of al-Bab from so-called Islamic State.
It is part of a broader offensive by Turkey to push IS and Kurdish fighters away from Turkey's southern border.
Russia and Turkey, who back opposing sides in the war, have been jointly targeting IS in air strikes recently.
A Russian aircraft attacked a building where some of the soldiers had been deployed near al-Bab, the Turkish military said.
It had been planning to hit IS targets but "by accident three of our soldiers were martyred when a building was bombed where our units were", it added.
Eleven other soldiers were wounded in the friendly fire incident on Thursday.
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 two leaders also agreed to "increase military co-operation during operations in Syria against IS militants and other extremist organisations".
The incident comes after a warming of ties between the two countries, which had previously been strained after Turkey downed a Russian jet near the Syrian border two years ago.
Turkish-backed rebel fighters have managed to capture the western outskirts of al-Bab, which lies about 30km south of Turkey. Turkish media say 10 Turkish soldiers have been killed in the fighting in recent days.
The Syrian rebels have been battling IS for the city since late 2016.
Syrian government forces, meanwhile, are also closing in on IS from the south, where fierce fighting is reported.
Reports suggest the rebels and government forces have reached an accommodation over al-Bab, orchestrated by Syria's key alley, Russia.
The two sides are now within 3km of each other on opposite sides of the city.