Three Turkish soldiers were “accidentally killed” on Thursday in a Russian airstrike during an operation against the Islamic State in Syria, Reuters reported. A bomb had hit the building occupied by the Turkish troops in the Islamic State-held town of Al-Bab. Eleven other soldiers were wounded.

The Turkish military in a statement acknowledged that the target was an accident on the Russian military’s part. “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 Turkish military said in a statement.

Russian President Vladimir Putin had called up his Turkish counterpart Tayyip Erdogan and expressed condolences. Putin also promised better coordination in the future to avoid such incidents.

The Kremlin said the incident took place owing to the “lack of agreement of coordinates during strikes by the Russian air force”, AFP reported. After Thursday’s airstrike, the number of Turkish troops killed in Syria has reached 64, according to AP.

Russia and Turkey have had a conflicted relationship as both sit on opposing sides in the Syrian conflict. While Moscow supports Syrian President Bashar al-Assad, Ankara had extended support to the rebels who oppose him. The relations soured even further in 2015, when the Turkish troops shot down a Russian air force jet that they claimed had crossed into Turkish airspace. However, the two countries have since appeared to reconcile and come together for the Syrian ceasefire deal.