At least 33 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 later retaliated against Syrian troops government targets.
Syrian forces supported by Russia are trying to retake Idlib from rebels who are backed by Turkish soldiers.
The Turkish military began hitting back at the Syrian targets after President Recep Tayyip Erdogan held an urgent top-level security meeting in Ankara late on Thursday.
Defence Minister Hulusi Akar and senior Turkish military commanders immediately went to the Syrian border to direct a wave of ground and air attacks against Syrian President Bashar al-Assad’s forces.
President Erdogan 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.
Nato Secretary General Jens Stoltenberg spoke by phone to Foreign Minister Mevlut Cavusoglu of Turkey, a key member of the military alliance.
Mr Stoltenberg “condemned the continued indiscriminate air strikes by the Syrian regime and its backer Russia in Idlib province”, his spokesperson was quoted as saying by the AFP news agency.
A spokesman for the US state department said in a statement: “We stand by our Nato ally Turkey and continue to call for an immediate end to this despicable offensive by the Assad regime, Russia and Iranian-backed forces.
“We are looking at options on how we can best support Turkey in this crisis.”
Meanwhile, UN Secretary General António Guterres expressed “grave concern” over the latest escalation, calling for an immediate ceasefire.
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.