It opens the door to a further escalation in Syria's messy northern battlefront where Turkey, Syrian rebels, the army, Iran-linked militias backing the Damascus government, Kurdish fighters, Russia and the United States are all present.
The Britain-based Syrian Observatory for Human Rights, said Turkey's month-long offensive against Afrin, backed by Syrian rebel groups, continued and included shelling of the main town there overnight.
Ankara aims to drive the Kurdish YPG militia from Afrin, seeing it as linked to an insurgency inside Turkey and as a threat to its border.
On Tuesday, militias that back Syrian President Bashar Assad arrived in Afrin at the Kurds' invitation to help thwart the Turkish assault, and Turkey and its allies tried to force them back with artillery fire.
A commander in the pro-Assad alliance said late on Tuesday that the militias had returned fire on the Syrian rebels fighting alongside Turkey.
According to Reuters, a spokesman for Turkish President Tayyip Erdogan stated Wednesday that pro-Assad forces retreated to east of Aleppo, and warned that an attempt at entering Afrin will have serious consequences.
The new phase of confrontation, pitting the Turkish army and its allies directly against pro-Assad forces, further scrambles the complex web of alliances and rivalries already at play in northern Syria.
Want to enjoy 'Zen' reading - with no ads and just the article? Subscribe todaySubscribe now