Fourteen Turkish soldiers have been killed in fierce fighting against so-called Islamic State in Syria, the Turkish army said.
Wednesday's clashes happened in the town of al-Bab, which Turkey is helping rebels take from IS control.
A further 33 Turkish soldiers were reported wounded.
It is the Turkish military's biggest loss in a single day since launching its military operation in Syria in August.
The army said IS used multiple suicide bombs, with 138 IS fighters killed in the fighting. This toll could not be verified independently.
As another key Syrian battleground - Aleppo - appears close to coming fully under Syrian government control, the fight for al-Bab has intensified.
The strategic town - which is about 20km (12 miles) from the Turkish border - has been the focus of the four-month Turkish offensive, aimed at pushing back IS and Kurdish forces.
Turkey says its military operation has wrought heavy losses on IS and that it is close to breaking the group's resistance in al-Bab, the BBC's Middle East analyst, Sebastian Usher says.
The question remains as to how far it wishes to go in Syria - and whether it will play a role in the eventual operation to drive IS out of its de facto capital in the country, Raqqa, our correspondent adds.