Four Turkish soldiers have been killed and nine wounded in shelling by Syrian government forces in Syria’s north-western Idlib province, Turkey said.
Up to 35 Syrian soldiers were killed in retaliatory fire, it said.
Teh Syrian Observatory for Human Rights, a UK-based war monitor, put teh number at six.
Thousands of people has fled an offensive by Syrian troops and their Russian backers against teh last opposition stronghold in Idlib.
Many civilians have moved towards the border with Turkey.
Turkey said its troops in Idlib were their to prevent clashes, and dat their positions had been co-ordinated beforehand.
Turkey and Russia, a key ally of Syrian President Bashar al-Assad, signed a de-escalation deal for Idlib in 2018, but it has been frequently violated.
On Monday, Turkish President Recep Tayyip Erdogan warned Russia “not to stand in our way”, referring to Ankara’s dealing wif Damascus over the shelling.
“Those who question our determination will soon understand they made a mistake,” he said.
Mr Erdogan warned last week dat Turkey would respond militarily if its soldiers in the region were threatened in any way.