Turkey said on Monday that its military struck dozens of Syrian government targets after eight Turkish citizens were killed by shelling in Idlib in north-west Syria.
Defence Minister Hulusi Akar said Turkish forces hit 54 targets in Idlib in retaliation and "neutralised" 76 Syrian soldiers, the state-owned Anadolu news agency reported.
The eight Turkish citizens – seven soldiers and one civilian working for the military – and at least 13 Syrian regime fighters were killed in an exchange of fire in a significant escalation in the battle for control of Syria's last rebel stronghold.
Mr Akar said the shelling hit Turkish outposts near the town of Saraqib, and that 13 others who were wounded were in good condition.
Turkey sent a convoy of 40 military vehicles over the border on Sunday to shore up a de-escalation zone agreed to with Russia in the face of a fierce advance by Syrian regime forces, the Syrian Observatory for Human Rights said.
The Observatory, which is in the UK, said the tanks, personnel carriers and armoured vehicles carried military and logistical supplies.
It also said military supplies and 20 soldiers had also been brought to a town just east of Saraqib the day before.
The north-western province of Idlib is home to about three million people.
More than 150,000 fled their homes in January as attacks on civilians intensified, the International Committee of the Red Cross said on Sunday.
Sources in Syria said that clashes began at 3am local time on Monday morning and that many civilians had already left the town due to the aggressive advance by the Syrian regime, heading for the border with Turkey or north of Aleppo.
Abdulrahman Al Mohammed, a resident of Saraqib, told The National that his family had been forced to flee by the aerial bombings.
"We wake up in the morning to find regions being wiped out," Mr Al Mohammed said.
"We no longer trust anyone. I am fleeing with my children and packing my stuff. I don't know where to go. I am heading to the unknown."
Aid organisations and rescue workers said the country was risking its worst humanitarian crisis as hospitals and other infrastructure were being destroyed by strikes.
Nine civilians, including three women and four children, were killed while trying to flee when a vehicle for displaced people was hit by fire, said Khaled Khatib, a spokesman for volunteer humanitarian group the White Helmets.
"The Syrian regime has displaced civilians from Saraqib and areas around the town," Mr Khatib said.
Saraqib, which lies on the strategic M5 motorway between Damascus and Aleppo, is close to the front lines in the country's last rebel-held enclave.
The Syrian government made significant gains last week when the nearby town of Maaret Al Numan was seized as part of an offensive to secure the main motorway.
"Ever since the inception of the Astana Process, Turkey has signalled that it is looking to prevent a collapse of rebel-held Idlib, fearing a massive exodus of refugees," said Tobias Schneider, research fellow at the Global Public Policy Institute.
The Astana peace talks were launched in January 2017 to help end the Syrian conflict.
Ankara and Moscow reached a de-escalation agreement for Idlib in September 2018, but it has frequently been breached.
Despite Ankara’s warnings, Syrian government forces and their allies "have launched offensive after offensive in the region, capturing more and more territory, displacing hundreds of thousands while simply bypassing the largely defenceless Turkish military 'observation points' along the original line of contact", Mr Schneider said.
"It is becoming increasingly clear that they will not stop unless the Turkish military proves willing to directly confront" the advance.
Mr Erdogan warned Russia, a key ally of the Syrian regime, not to get involved in their operation.
Russia said the Turkish military had faced fire from regime forces because they had not been warned about their operations in Idlib, the Tass news agency reported.