The YPG militia is regarded by Ankara as the Syrian affiliate of the banned Kurdistan Workers' Party (PKK), listed as a terrorist organization by Turkey.
Turkey is targeting the production of unmanned tanks for its armed forces, President Recep Tayyip Erdoğan has stated.
Syrian state news agency SANA reported that on February 19 president Assad had agreed to send forces to aid the YPG in Afrin after the Kurds appealed for help to protect the northern border and Turkey announced it would besiege Afrin's capital.
"We warn the Turkish leadership that if they initiate combat operations in the Afrin area, that will be considered an act of aggression by the Turkish Army,"Syrian Deputy Foreign Minister Faisal Meqdad said in a statement reported by state media".
The Syrian Kurds deny any direct organisational links to the PKK - an assertion backed by the USA, which has provided the militia and allied Arab fighters with weapons and air support to help them battle the jihadist group Islamic State (IS).
Turkey warned on Wednesday that pro-Syrian regime forces entering the Afrin region become targets of the Turkish forces in the area, according to a Reuters report.
Istanbul launched the military offensive called "Olive Branch" on January 20 with the aim of dismantling the Kurdish People's Protection Units (YPG) in Syria's northern Afrin region, where Kurds have set up three autonomous cantons.
Afrin is just one of many active fronts in the country's civil war, which began as a popular uprising against President Bashar al-Assad in 2011.
He added that the militias were forced to go back after artillery shooting.
Meanwhile, Russia had called for direct negotiations between Damascus and Ankara to resolve the Afrin crisis.
The UN says more than 300,000 people have been displaced by the fighting in Idlib since December, while nearly 200 civilians are reported to have been killed in government air and artillery strikes on the Eastern Ghouta since Monday.