The U.S.-backed People's Protection Units (YPG), the Syrian affiliates of the PKK terrorist group, clashed with regime fighters in the divided northeastern city of Qamishli on Saturday, leading to the deaths of 18 combatants.
The rare flare-up in the city, which is largely held by the YPG's political entity the Democratic Union Party (PYD), near the Turkish border saw 11 regime fighters and seven YPG militants killed, Asayish, the PYD's police force, said.
The Syrian Observatory for Human Rights, a Britain-based war monitor, reported the same death toll.
The Asayish said a regime "patrol opened fire on our forces with light and medium weapons, causing our forces to respond to this violation, which killed 11 regime fighters... and seven of our comrades".
The Observatory said the shootout started when Asayish members at the checkpoint asked regime fighters aboard a patrol vehicle to step out but they refused.
"When they did not comply, the shooting started on the car," Observatory chief Rami Abdelrahman said, and the clashes escalated after both sides called in reinforcements.
An AFP correspondent at the scene saw empty camouflaged pick-up trucks in the street.
Some bore bullet holes, while traces of blood were visible on the tarmac, he said.
The YPG controls most of Qamishli, but regime forces and allied militiamen hold part of the city and its airport.
Deadly clashes last broke out between the YPG and pro-regime fighters in Qamishli in April 2016, but ended days later with a truce.
The Damascus regime has vowed to reintegrate the YPG-held areas, by force if necessary.
In late July, it opened talks with the U.S.-backed Syrian Democratic Forces, which is dominated by the YPG, on a negotiated settlement.
The YPG has organic organizational and operational links with the PKK, a group considered a terrorist organization by the U.S., the EU and Turkey. Despite the links, Washington has picked the YPG as its partner in its fight against Daesh in Syria, supplying the group with weapons and other military equipment.
In late July, the YPG conducted the first step of negotiations with the Assad regime, reportedly agreeing on a decentralized state system for Syria and handing over key cities, including Raqqa and Deir el-Zour, to the regime.
The YPG's ultimate aim is to establish an autonomous region in northern Syria by connecting the northwestern Afrin canton to the Kobani and Jazeera regions in the northeast. However, with Turkey's successful operation in Afrin and cooperation on Manbij with the U.S., the group's autonomy plans, which Ankara terms a "terror corridor," won't pan out.
Pundits have claimed that the YPG's efforts on reaching a deal with Assad were due to the recent steps in Manbij between the U.S. and Turkey, which has proposed the withdrawal of the group from the province.