President Vladimir Putin has sent condolences after a Russian air strike accidentally killed three Turkish soldiers in northern Syria.
The strike hit a building near the town of al-Bab, believing it to contain Islamic State (IS) fighters rather than Turkish troops, Turkey's military said.
Eleven others were wounded in the so-called friendly fire incident.
They were supporting Syrian rebels who are locked in a fierce battle to try to capture al-Bab from IS fighters.
Russia and Turkey, who back opposing sides in the Syrian war, have been jointly conducting air strikes on IS in the town in recent weeks.
'Tragic incident'
The Russian aircraft 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," Turkey's military said.
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".
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President Putin told Recep Tayyip Erdogan that poor co-ordination between Moscow and Ankara was to blame for the accident, RIA news agency reports.
The two leaders agreed to "increase military co-operation during operations in Syria against IS militants and other extremist organisations".
Both countries are conducting a joint investigation, Turkey's military said.
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.
Earlier on Thursday, Turkish-backed rebel fighters managed to capture the western outskirts of al-Bab, which lies about 30km (20 miles) south of Turkey. Turkish media say 10 Turkish soldiers have been killed in the fighting in recent days.
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