At least 22 Turkish solders have been killed in an air strike by Syrian government forces in Syria's northwestern Idlib region.
Governor of the southeastern Turkish province of Hatay, Rahmi Dogan, said the death toll from the strike was initially nine soldiers.
He later confirmed 22 Turkish solders died in the attack but the Syrian Observatory for Human Rights puts the figure at 34 solders dead.
The deaths mark a serious escalation in the direct conflict between Turkish and Russia-backed Syrian forces that has been waged since early February.
According to the latest reports in the New York Times, the attack could "dramatically shift the course of the Syrian war".
Posted by Syrian Observatory for Human Rights on Thursday, February 27, 2020
Turkish President Recep Tayyip Erdogan called an emergency security meeting in Ankara, broadcaster NTV reported.
Foreign Minister Mevlut Cavusoglu spoke to NATO Secretary-General Jens Stoltenberg by telephone, according to state-run Anadolu news agency.
The Times reported a convoy traveling to resupply one of 12 Turkish posts set up a year ago came under attack and then the post itself was bombed, a Turkish-backed Syrian official part of the fighting force in Idlib province told the paper in an interview.
"No Syrian fighters were hurt in the bombing. The resupply convoy and the post were solely Turkish," he said.
Fighting continued earlier throughout the week when Turkish-backed Syrian fighters captured the town of Saraqib on the main M5 highway through the province on Wednesday, and on Thursday were engaged in fierce battles further south.
The casualties mark the largest death toll for Turkey in a single day since Ankara first intervened in Syria in 2016.
At least 43 have now been killed in Idlib since the start of February.
The latest air strike came after a Russian delegation spent two days in Ankara for talks with Turkish officials on the situation in Idlib, where a Syrian government offensive has sent hundreds of thousands of civilians fleeing towards the Turkish border.
Assad now controls almost the entire southern part of Idlib province after capturing more than 20 villages on Thursday, state media and opposition activists said.
Tweet from @SyrianKurdPress
It is part of a weeks-long campaign backed by Russian air power into Syria's last rebel stronghold.
Thousands of Turkish soldiers are deployed inside rebel-controlled areas of Idlib province, which is dominated by al-Qaeda-linked militants.
The incident came as a senior Turkish official told Reuters the government would no longer stop Syrian refugees reaching Europe.
In anticipation of the imminent arrival of refugees from Idlib, Turkish police, coast guard and border security officials have been ordered to stand down on refugees' land and sea crossings, the official said.
Turkey hosts about 3.7 million Syrian refugees. Under a deal agreed in 2016, the European Union has provided billions of euros in aid in return for Ankara agreeing to stem the influx of migrants into Europe.