Turkey fired at Syrian pro-regime forces as they entered Tuesday the Kurdish-controlled enclave of Afrin, Syrian state media said.
The shelling marks a major escalation in the month-old assault Turkey and allied rebels are waging on Afrin.
"Turkish regime forces targeted the locations of popular forces with artillery fire as they arrived to the Afrin region," state news agency SANA reported.
Turkey said they it fired "warning shots" at Syria pro-regime forces in Afrin.
"Pro-regime terrorist groups that are trying to advance towards Afrin retreated to about 10 kilometres (six miles) from the town because of the warning shots," Turkish state news agency Anadolu said.
Hundreds of pro-government fighters arrived on Tuesday afternoon to Afrin, to take a stand against a Turkish-led offensive on the region.
Their deployment marks the first time since 2012 that regime forces take up positions in the area, which is held by the Kurdish People's Protection Units (YPG).
In a statement on Tuesday, YPG spokesman Nuri Mahmud said the Kurdish forces had called on the Damascus government to help fend off Turkey's assault.
"The Syrian government responded to the invitation, answered the call of duty and sent military units today, February 20, to take up positions on the borders, and participate in defending the territorial unity of Syria and its borders," the statement said.
AFP correspondents said the forces did not appear to have entered Afrin city itself.
The YPG has controlled Afrin since Syrian government forces withdrew from Kurdish-majority areas in the country's north in 2012.
But on Monday, Syria's state media said that pro-Damascus forces would head to Afrin to "join the resistance against the Turkish aggression".
"This comes in the framework of supporting residents and defending the territorial unity and sovereignty of Syria," news agency SANA said.
The Afrin region lies in Syria's northern province of Aleppo.
An AFP correspondent in government-controlled Aleppo city told AFP that the convoy, made up of pick-up trucks carrying armed men, had left earlier Tuesday en route to Afrin.