Four Turkish soldiers have been killed and nine wounded in shelling by Syrian government forces in Syria’s north-western Idlib province, Turkey said.
It said up to 35 Syrian troops had been killed in retaliatory fire. A UK-based monitoring group put the number at six.
Thousands of people have fled an ongoing offensive by Syrian troops and their Russian backers against the last opposition stronghold in Idlib.
Many have moved towards the border with Turkey, which supports the opposition.
Turkey and Russia – a key ally of Syrian President Bashar al-Assad – signed a de-escalation deal for Idlib in 2017, but it has been frequently violated.
What did Turkey say?
Speaking to reporters on Monday morning, Turkish President Recep Tayyip Erdogan said 46 Syrian “regime targets” had been hit by the Turkish military.
He said that “30-35 Syrians on the other side have been neutralised.
“Those who question our determination will soon understand they made a mistake,” Mr Erdogan said.
He warned Russia not to get involved in Ankara’s dealing with Damascus over the shelling, telling Moscow “not to stand in our way”.
Turkey has previously said its troops were in Idlib to prevent clashes there, and that their positions were being co-ordinated beforehand.
Mr Erdogan warned last week that Turkey would respond militarily if its soldiers in the region were threatened in any way.
What’s the background to this?
In 2017, Turkey and Russia signed a “de-escalation” deal on Idlib, which came into force a year later.
The two sides agreed to jointly patrol the area to prevent clashes between the opposition and Syrian government troops.
Turkey, which has 12 military observation posts in the region, has accused Russia of violating the agreement, a claim Moscow denies.
There are 3.6 million Syrian refugees in Turkey, and the president has said it would not be able to handle a fresh influx of displaced people.