Six Turkish soldiers have been killed and nine wounded in shelling by Syrian government forces in Syria's north-western Idlib province, Turkey said, reports BBC. It said up to 35 Syrian troops had been killed in retaliatory fire. Syria's media said there were no casualties.
Hundreds of thousands of people have fled an 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. 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 [killed]. "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 dealings 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. Syria's Sana state news agency said that "no injuries or damage were reported" in the Turkish strikes. It said Syrian government troops were continuing their offensive in Idlib, "liberating" several villages in the province.
Meanwhile, the Syrian Observatory for Human Rights, a UK-based monitoring group, reported that six Syrian soldiers were killed in the Turkish strikes.
The strains are beginning to show in the relationship between Turkey and the Syrian regime, which in turn is souring ties between Ankara and Moscow. This was never going to be a marriage made in heaven, Turkey having funded and armed opponents of the government from the outset of the Syrian crisis with the explicit aim of removing President Bashar al-Assad from power.
However, when it became clear that President Assad's survival was assured, President Erdogan decided to change tack and sought out a better relationship with both Damascus and Moscow. This, he believed, was the best way to secure Turkey's own strategic interests in Syria. But Turkey's goals are incompatible with those of the other players.
Russia's backing for a Syrian government offensive in Idlib province suggests that Moscow is choosing its long-time Syrian ally over Ankara. The localised shelling between Syrian and Turkish forces indicates that the diplomatic tensions present from the outset are now expressing themselves in outright fighting on the ground.
For endangered civilians, many of whom have been displaced by the government offensive, it makes the end game in Idlib that much more dangerous. 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.