The curfew applies to the Sheikh Maqsud and Ashrafiyeh districts, both with large Kurdish populations, as well as four adjacent areas.
The two Kurdish-dominated areas have been targets of attacks the Syrian military has carried out in Aleppo in recent days.
Both on Wednesday and Thursday, the attacks were preceded by calls for civilians to leave. On Wednesday, local authorities said that over 40,000 people had left.
Sources within both the government and the Kurdish forces told AFP that at least 21 people have been killed in the past three days.
The clashes are taking place as political negotiations have stalled between Syria's new government, which came to power after the fall of dictator Bashar al-Assad at the end of 2024, and the Kurdish-led Syrian Democratic Forces (SDF).
In March, the government in Damascus signed an agreement with the SDF, which controls large parts of northeastern Syria, under which the SDF would merge with the country's regular army by the end of 2025. The fighting is rooted in disagreements over how this should happen, writes the AP news agency.
In a post on X on Thursday evening, US envoy Tom Barrack urged the parties to "pause the fighting" and added that Washington was following the developments with "grave concern". A few hours earlier, the EU urged government forces and the SDF to exercise restraint.
