At least 29 Turkish soldiers have been killed in an air strike by Syrian “regime forces” in north-western Syria, a senior Turkish official has said.
More were hurt in Idlib province, said Rahmi Dogan, the governor of Turkey’s Hatay province. Other reports put the death toll higher.
Turkey is now retaliating against Syrian troops government targets.
Syrian forces supported by Russia are trying to retake Idlib from rebels who are backed by Turkish soldiers.
The Syrian authorities have so far made no public comments on the latest escalation in Idlib, the last Syrian province to remain in opposition hands.
The Turkish military began hitting back at the Syrian targets after President Recep Tayyip Erdogan held an urgent top-level security meeting in Ankara.
The Turkish leader wants the Syrian government forces to pull back from positions where Turkey has set up military observation posts and had earlier threatened to attack them if they did not halt their advance.
But Syria’s government and Russia have rejected his demand to pull back to ceasefire lines agreed in 2018. Russia has also accused Turkey of violating the 2018 ceasefire by backing rebels with artillery fire.
The UK-based monitor, the Syrian Observatory for Human Rights, said at least 34 Turkish troops had been killed in Thursday evening’s air strike.
The wounded had been brought back to Turkey for treatment, Mr Dogan said.
“All known” Syrian government targets were under fire by Turkish air and land support units, Turkey’s communications director Fahrettin Altun was quoted by state news agency Anadolu as saying. Turkey had decided to “respond in kind” to the attack, Mr Altun said.
Meanwhile, Nato-member Turkey said Foreign Minister Mevlut Cavusoglu had spoken to Nato Secretary-General Jens Stoltenberg.
A huge new element of risk
By Sebastian Usher, BBC World Service Middle East editor
This is a new and dangerous escalation in an increasingly direct conflict between Turkish and Syrian government forces in Idlib. Both sides have suffered losses over the past few weeks. But the latest Turkish casualties come at a precarious moment.
President Erdogan has threatened to mount a major military operation against President Assad’s forces if they don’t pull back from frontline positions near Turkish troops in Idlib within the next two days.
For now there’s no sign of that happening. Turkey has already been stepping up its military support for the rebel fighters it backs as they mount a counter offensive to try to win back key towns they’ve recently lost.
Behind this conflict looms the potential of an even bigger confrontation. Turkey and Russia have backed different sides in Syria, but have come together to broker battlefield deals in the past few years.
That pragmatic rapprochement is now in doubt. Russian airpower has provided vital support for Syrian forces – if it is now being directed at Turkish military positions, that creates a huge new element of risk.
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 in Idlib has driven nearly a million Syrians from their homes since December. The UN said a full-scale battle there could result in a “bloodbath”.
Reuters news agency quoted a senior Turkish official on Thursday as saying that Turkey had decided to stand down its border guards and no longer prevent Syrian refugees from trying to reach Europe. However, this has not been officially confirmed.
Earlier on Thursday, Mr Erdogan said three Turkish soldiers had been killed in an air strike in Idlib.
Turkey’s defence ministry said it had responded to that incident by hitting Syrian “regime targets”.
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.