Kurdish forces in the enclave requested help against advancing Turkish troops
Syrian pro-government forces have entered the northwestern city of Afrin, despite shelling by Turkish troops trying to keep them out.
The new arrivals have come to help local Kurdish fighters fend off an offensive by Turkey.
Kurdish leaders say they had to turn to President Assad and his allies as no foreign powers would help them against advancing Turkish forces.
"We came to the city of Afrin to support our brothers here, and now we are standing in the heart of Afrin, as you see," a commander of the pro-government Syrian forces said, as comrades celebrated.
"We are in the heart of Afrin with huge numbers and we will stay here until we kick out the Ottoman Turk occupiers."
The pro-government forces entered the city despite an uneasy relationship between Damascus and the YPG Kurdish milita. Kurds seek autonomy in regions they hold while Assad wants to control all of Syria.
For Turkey's President Recep Tayyip Erdogan, the priority remains driving back the YPG which Ankara sees as a threat along its border, with links to the Kurdish insurgency at home.
Erdogan described the convoy of Syrian forces as being made up of 'terrorists' acting independently.
But in Britain's Parliament on Tuesday, the Turkish offensive was condemned by the opposition Labour Party.