Syrian state media said on Monday that pro-government forces were set to enter the northern Kurdish enclave of Afrin to end Turkey's ongoing offensive there.
“Popular forces will enter Afrin within hours,” reported Al Ikhbariya, Syrian state television.
The move may prompt Turkey to curtail its weeks-long ground assault against the Kurdish-dominated Syrian Democratic Forces alliance, but also creates the potential for more fighting and military miscalculations.
Pro-government forces and Kurdish-led fighters have fought each other elsewhere in Syria, and Damascus opposes the Kurds’ demands for autonomy. But in Afrin they have a common enemy and a mutual interest in putting a stop to Ankara's offensive.
Turkey's Foreign Minister Mevlut Cavusoglu said on Monday that Syrian forces could not stop the Turkish army's advance.
"If the [Syrian] regime is entering there to cleanse the PKK and PYD, then there are no problems," said Mr Cavusoglu, referring to the Kurdistan Workers' Party, a Turkish Kurdish insurgent group, and the Syrian Kurdish Democratic Union Party respectively.
But "if it comes in to defend the YPG, then nothing and nobody can stop us or Turkish soldiers," Mr Cavusoglu adding during a news conference in Jordan.
The PYD's armed wing, the People's Protection Units (YPG), is the dominating force in the SDF alliance. Ankara accuses the YPG of having links to the PKK, which has been waging a decades-long insurgency in Turkey.
Kurdish forces have yet to confirm any imminent deployment of Syrian forces to Afrin.
But on Sunday an official from the PYD said Syrian government forces would enter Afrin on Monday following a deal struck between Damascus and the SDF.
Badran Jia Kurd, an adviser to the Kurdish-led administration that runs swathes of northern Syria, said the deal was purely military and that any political agreement would have to await further negotiations between Damascus and the Kurdish administration.
Turkish officials said President Recep Tayyip Erdogan had held a telephone conversation with his Russian counterpart during which the two men reaffirmed their determination to co-operate in the fight against "terrorism" in Syria.
Mr Erdogan and Vladimir Putin also discussed Turkey's military offensive in Afrin and the two countries' efforts to set up observation posts in Syria's northern Idlib province.
The Syrian Observatory for Human Rights, a Britain-based war monitoring group, said Turkish forces and their Syrian Arab allies shelled villages in Afrin on Friday. Citing medical sources in Afrin, it reported that six people suffered breathing difficulties and dilated pupils following the shelling, indicating a suspected gas attack.
Sunday's reported deal highlights the complex situation in northern Syria, driven by rivalries and alliances between Kurdish forces, the Syrian regime, rebel groups, Turkey, the United States and Russia.
Turkey's offensive in Afrin, dubbed "Olive Branch", has seen Ankara sending troops into Syria and pounding the region with air strikes and artillery fire.
Damascus has denounced Ankara's "aggression" but until Monday had not explicitly said it would intervene.
Also on Monday, the Observatory said regime bombardment of the rebel-held Damascus suburb of Eastern Ghouta had killed at least 35 civilians in the last 24 hours.
"The regime is bombing Eastern Ghouta to pave the way for a ground offensive," said Observatory head Rami Abdel Rahman.
Syrian President Bashar Al Assad has dispatched reinforcements to Eastern Ghouta in recent weeks in an apparent bid to retake the suburb, now the last opposition-controlled pocket around Damascus.