The leadership in Damascus signed a deal with the SDF in March for it to merge with the Syrian army by the end of 2025 but there have been disagreements over how that would happen.
Clashes between government and Kurdish forces in a contested area of the northern Syrian city of Aleppo intensified on Thursday after authorities ordered civilians to evacuate.
Syrian authorities opened a corridor for civilians to evacuate for a second day and tens of thousands fled the city.
The government of Aleppo province gave residents until 1 pm local time to leave in coordination with the army.
State news agency SANA, citing the army, said the military would begin "targeted operations" against the Kurdish-led Syrian Democratic Forces (SDF) in the neighbourhoods of Sheikh Maqsoud, Achrafieh and Bani Zaid half an hour after the deadline.
The military later issued a series of maps with the areas subject to evacuation orders.
More than 142,000 people have been displaced across the province, according to the Aleppo Central Response Committee.
"There's a large percentage of them with difficult medical issues, elderly people, women, and children," said Mohammad Ali, operations director with the Syrian Civil Defence in Aleppo.
Kurdish forces said at least 12 civilians were killed in the Kurdish-majority neighbourhoods, while government officials reported at least nine civilians have been killed in the surrounding government-controlled areas in the fighting that broke out Tuesday.
Dozens more on both sides have been wounded. It was not clear how many fighters were killed on each side.
Each side has accused the other of deliberately targeting civilian neighbourhoods and infrastructure, including ambulance crews and hospitals.
Clashes intensified in the afternoon, with continuous exchanges of shelling and drone strikes, and tanks could be seen rolling into the contested neighbourhoods. The SDF-affiliated Internal Security Forces said they had "destroyed two armoured vehicles and inflicted casualties on the attackers" as they advanced.
Aleppo governor Azzam al-Gharib said on Thursday evening that "a large number" of SDF fighters had defected or fled and that security forces were preparing to deploy in the area.
Political impasse
The clashes come amid an impasse in political negotiations between the central state and the SDF.
The leadership in Damascus under interim President Ahmed al-Sharaa signed a deal with the SDF in March, which controls much of the northeast, for it to merge with the Syrian army by the end of 2025.
There have been disagreements on how it would happen. In April, scores of SDF fighters left Sheikh Maqsoud and Achrafieh as part of the deal.
Officials from the central government and SDF met again on Sunday in Damascus, but government officials said that no tangible progress had been made.
Some of the factions that make up the new Syrian army, formed after the fall of former President Bashar al-Assad in a rebel offensive in December 2024, were previously Turkey-backed insurgent groups that have a long history of clashing with Kurdish forces.
International concerns
The SDF has for years been the main US partner in Syria in fighting against the so-called Islamic State (IS) group, but Turkey considers the SDF a terrorist organisation because of its association with the Kurdistan Workers’ Party (PKK), which waged a long-running insurgency in Turkey. A peace process is now underway.
Despite the long-running US support for the SDF, the Trump administration has also developed close ties with al-Sharaa's government and has pushed the Kurds to implement the March deal.
A US State Department official said in a statement on Thursday that the US "is closely monitoring the situation" and urged "restraint on all sides."
It said US envoy Tom Barrack is trying to facilitate dialogue between the two sides.
"All parties should focus on how to build a peaceful, stable Syria that protects and serves the interests of all Syrians, rather than pushing the country back into a cycle of violence," the statement said.