The Syrian Observatory for Human Rights says a government airstrike on a Kurdish district in the city of Aleppo has left 15 people dead, including nine children.
The organisation said in London that southern Damascus and Qadam in the south-west were also targeted by the Syrian air force on Saturday.
It said Kurdish fighters later killed five soldiers in a revenge attack on an army checkpoint.
Meanwhile, President Bashar al-Assad has told Turkish television that a rebel victory might destabilise the Middle East for decades.
"The situation will inevitably spill over into neighbouring countries and create a domino effect throughout the Middle East and beyond ... spreading east, west, north and south.
''This will lead to a state of instability for years and maybe decades to come," he said.
The uprising began two years ago. Rebels hold several eastern and southern districts of Damascus and pose a growing challenge in the southern province of Deraa. They also hold territory in the north and east of the country.