Turkey hit targets in northern Syria, responding to shelling by Syrian government forces that killed at least six Turkish soldiers, the Turkish president said Monday.
A Syrian war monitor said 13 Syrian troops were also killed.
The escalation comes amid a Syrian government offensive into the country’s last rebel stronghold, located in Idlib and parts of the nearby Aleppo region.
Turkish troops are deployed in some of those rebel-held areas to monitor an earlier cease-fire that was agreed to but that has since collapsed.
Relations between Turkey and Syria have deteriorated sharply since Syria’s civil war began in 2011.
Syria accuses Turkey of undermining its security by allowing thousands of foreign fighters to come battle the Syrian army.
Idlib province is currently dominated by al-Qaida-linked militants.
This recent development is likely to further increase tensions between the two neighboring countries as such direct clashes have been rare.
It could also cause friction between Moscow and Ankara, which have sought to coordinate their actions in Syria.
Erdogan said Turkey had told Russian counterparts “they need to stand aside” in the escalating conflict, in which Ankara and Moscow back opposing sides.
“It is not possible for us to remain silent when our soldiers are being martyred,” Erdogan said.
He accuses Russia of violating a 2017 “de-escalation” agreement to reduce fighting in the region, a charge Moscow denied on Friday.
Russia’s Defense Ministry said Turkish military units came under fire overnight after moving within Idlib without notifying Russia, contradicting Ankara’s claim that it coordinated movements.
The province of Idlib is home to some 3 million people, many of them displaced from other parts of Syria in earlier bouts of violence.
The United Nations has estimated that about 390,000 Syrians have been displaced there over the past two months — 315,000 in December and 75,000 in January.
Turkey, which already hosts 3.6 million refugees from Syria, fears a fresh wave of migrants from Idlib.
It has 12 military observation posts around the region, set up under a 2017 agreement with Russia and Iran.