At least 22 Turkish soldiers have been killed in an airstrike by Syrian “regime forces”, a Turkish provincial governor has said.
Several more were badly hurt in the attack in Idlib in the north-west, Hatay governor Rahmi Dogan said. Other reports put the death toll higher.
Turkey’s president held a top-level security meeting after the strike.
Syrian forces are trying to retake Idlib from rebels who are backed by Turkish soldiers.
Turkish President Recep Tayyip Erdogan wants Syrian forces to pull back from positions where Turkey has set up military observation posts.
Turkey has also threatened to attack Syrian government forces if they do not halt their advance.
Earlier on Thursday, Mr Erdogan said three Turkish soldiers had been killed in an airstrike in Idlib.
Turkey’s defence ministry said it had responded to that incident by hitting Syrian “regime targets”.
Media captionWatch: Turkey’s defence ministry reiterates his country’s threat to Syria
The latest clashes came after the Turkey-backed rebels said they had retaken the strategic town of Saraqeb from Syrian government forces on Thursday.
The fighting has driven nearly a million Syrians from their homes since December. The UN said a full-scale battle for Idlib could result in a “bloodbath”.
But Russia has rejected calls in the UN Security Council for a humanitarian ceasefire in northern Syria.
Responding to a statement from Belgium and Germany that the killing of civilians must stop, the Russian ambassador said the only solution was to chase what he called the terrorists from the country.
The Russian defence ministry on Thursday accused Turkey of violating a peace deal for Idlib by backing rebels with artillery fire.