At least 33 Turkish soldiers have been killed in an air strike by Syrian government forces in north-west Syria, a senior Turkish official has said.
Others were injured in the attack in Idlib province, said Rahmi Dogan, the governor of Turkey's Hatay province.
Turkey retaliated by attacking Syrian government targets.
Russia, Syria's key military ally, said the Turkish troops had been operating alongside jihadist fighters when they were attacked by Syrian forces.
Government forces, supported by Russia, have been trying to retake Idlib from rebels who are backed by Turkey. The air strike came after the rebels retook the key town of Saraqeb.
The Syrian authorities themselves have so far made no public comments on the latest escalation in Idlib, the last Syrian province to remain in opposition hands.
Reports suggest Turkey, a key member of the Nato alliance, may be relaxing its border controls to allow Syrian refugees to seek refuge in the EU.