Turkey gave Syria an ultimatum on Wednesday to pull its forces back from its military posts in the country's last rebel enclave following unprecedented clashes between their forces this week.
The escalation between Turkish and Syrian troops -- which saw more than 20 people killed in exchanges on Monday -- is testing the uneasy relations between Turkey and Russia, the key foreign brokers of the conflict.
"If the regime does not pull back, Turkey will be obliged to take matters into its own hands," Turkish President Recep Tayyip Erdogan told a meeting of his ruling party MPs in Ankara, giving Syria until the end of the month to comply.
But Syrian regime forces were continuing their offensive in northwestern Idlib province on Wednesday -- one which has killed 300 civilians since December and displaced some 520,000 people in one of the biggest upheavals of the nine-year war.
Syrian troops have seized more than 20 towns and villages from rebels and jihadists over the past 24 hours, according to the Britain-based Syrian Observatory for Human Rights and state news agency SANA. Erdogan said two of Turkey's 12 observation posts in Idlib, set up under a 2018 agreement with Russia, were now "behind the regime's lines".
The post at Morek was surrounded by Assad's forces in December, while another at Surman now lies within the regime's area of control. Eight Turkish troops and civilians were killed on Monday by regime fire in Idlib and Turkey killed at least 13 Syrian government troops in response, according to monitors, in the bloodiest clashes since Ankara sent troops to Syria in 2016.
Erdogan called on Moscow, the key backer of President Bashar al-Assad's regime, to "better understand our sensitivities in Syria". He spoke with his Russian counterpart Vladimir Putin on Tuesday and told him Ankara would respond "firmly" to any new Syrian attack.
Syria's military command said Tuesday that the presence of Turkish forces was "illegal and a flagrant act of aggression", vowing to respond to any Turkish attack on its forces, state news agency SANA reported.
Erdogan said the clashes amounted to a "new era" in Syria, and that any further attacks would be "responded to in kind". "The air and ground elements of the Turkish armed forces will freely move in the Idlib region and if needed will launch an operation," he said.
The mass displacement of civilians in Idlib has coincided with a biting winter and is one of the largest since the 2011 start of a conflict that has seen more than half of Syria's pre-war population of 20 million displaced. Turkey, which already hosts some 3.7 million Syrian refugees, wants to prevent a further influx.
The latest clashes have tested the tricky balancing act between Turkey and Russia, who support opposing sides but have attempted to broker political solutions in Idlib. Under a deal reached last year, Russia was to prevent Damascus from launching a devastating operation in Idlib, in exchange for Turkey containing extremist groups running the region, but its proxies were largely sidelined by Syria's former Al-Qaeda affiliate, Hayat Tahrir al-Sham.
Jana Jabbour, of Sciences Po in Paris, said the latest tensions would not derail Russia and Turkey's cooperation. "(They) know perfectly how to manage their differences. They need to cooperate and maintain good relations because they are economically interdependent," she told AFP. Their priority, she added, is to ensure a "regional solution, designed by Moscow and Ankara, as opposed to an international solution parachuted in by the West".