Kurdish forces backed by the United States made gains against militias loyal to Syrian President Bashar al-Assad, the pro-Kurdish Firat news agency reported Friday, amid renewed clashes and a fresh wave of airstrikes.
The Syrian Observatory for Human Rights said 23 civilians were killed in the clashes since Thursday, among them nine children, along with five Kurdish fighters and four pro-government militia members. Scores are fleeing the worst-affected areas.
The clashes in al-Hassakeh city in north-eastern Syria have been ongoing for most of the week and saw the Syrian airforce on Thursday conduct airstrikes against the Kurds for the first time in the history of the five-year civil war.
Territory on the outskirts of the city was changing hands with Firat claiming Kurdish forces had made gains. SANA, the Syrian government's news agency, did not report on the clashes.
Firat and the Observatory also reported another wave of several airstrikes carried out by government forces on Friday in the city. There was no confirmed casualty toll.
The Kurdish People's Protection Units (YPG) have denounced the government and its local militias, who are the main al-Assad force on the ground in al-Hassakeh engaged in the fighting.
Analysts have recently suggested that the government's control over loyalist militias in certain parts of the country is slipping after years of war.
Citing recent gains by YPG-led forces against Islamic State - including the seizing of the strategic city Minbij, just along the Turkish border, which should help cut the flow of foreign fighters - the Kurds accused al-Assad of "backing" the extremist group.
Meanwhile, the Democratic Forces of Syria (DSF), which is dominated by the YPG, said it was handing over control of Minbij to local forces. Turkey, wary of Kurdish gains, is demanding the YPG leaves Minbij. Before the war, the city was mixed, with an Arab majority.
Observatory head Rami Abdurrahman said the handover to the Minbij Military Council - a composite force of different local groups set up months ago before the push to take the city - was made at the request of the US and in order to placate Turkey.
Turkey's state-run news agency Anadolu quoted Foreign Minister Mevlut Cavusoglu as being pleased with the handover.
The DSF also published three photos, taken from the air, which show large convoys leaving Minbij. The US-backed force said the photos were taken last week and pictures the Islamic State forces retreating, while using human shields.
The US and DSF had repeatedly accused Islamic State of hiding behind civilians. It had been reported the group's last fighters were granted safe passage to retreat to a town on the Turkish border last week, as they were holding thousands of civilians hostage.
For most of the war, the YPG and the Syrian government have managed to make tacit agreements and avoid direct conflict. The government's forces abandoned many Kurdish parts in the north of the country in the early stages of the conflict.
The Kurds, who make up about 10 per cent of Syria's pre-war population of 22.4 million, long complained of discrimination under the rule of al-Assad's Baath Party, an Arab nationalist organization. Since 2011, they have expanded their push for autonomy.
This demand for autonomy and willingness to cut deals with the al-Assad government have been among the factors contributing to a worsening of the already tense relationship between the Kurds and Arab-dominated rebel factions in Syria, some of which are Islamist.