Clashes erupted on Saturday between US-backed Kurdish fighters and Syrian troops in the center of the city of Qamishli in northeastern Syria that left at least 18 people killed, Kurdish forces said.
The fighting took place after a Syrian military convoy entered areas in the city which the Kurdish YPG militia's internal security forces said were under their control.
"They entered our areas of control and arrested civilians and members of the patrol targeted our forces," the internal security forces, known as the Asayish, said in a statement.
Kurdish forces said seven of its fighters and 11 Syrian military were killed in the clashes.
Pro-government sources told state media an army patrol was attacked by Kurdish forces while on its way to the airport. It said several troops were killed.
The Kurdish YPG militia, which spearheads the US-backed Syrian Democratic Forces (SDF), controls most of the city and pro-government forces holding the airport and part of its center.
Fighting in Qamishli, near the Turkish border, which erupts occasionally, disturbs a peaceful co-existence where the Syrian state has slowly expanded its influence, residents say.
Syrian President Bashar Assad has turned a blind eye to YPG control of Kurdish-populated cities since the 2011 uprising in which his army has focused on fighting mainly Sunni rebel factions seeking to topple his rule.
But the government has not ruptured ties with salaries of many state employees in these areas still paid and authorities still getting a share of proceeds from oilfields now under Kurd control.
The SDF has expanded beyond mainly Kurdish parts of the north, where the forces have carved out autonomous cantons since the onset of Syria's conflict.
The region they control spreads across much of northern and eastern Syria, which is rich in farmland, oil and water.
Senior members of the YPG have recently held talks with Syrian officials seeking a political deal which would retain their autonomy in Syria.