An air strike killed at least 12 people and wounded more than 30 in the northeastern Syrian town of Ras al-Ain on Monday, the Syrian Observatory for Human Rights said.
Among the dead were four civilians and eight rebels, the Britain-based watchdog said.
"The warplane hit the Mahata district in the southwestern part of the town. This area is controlled by the (jihadist) Al-Nusra Front, Ghuraba al-Sham and other rebel battalions," the Observatory's Rami Abdel Rahman told AFP by phone.
The northeastern sector of the town, located in the mainly Kurdish province of Hasakeh, is controlled by Kurdish militias, he noted.
"A MIG aircraft made two consecutive bombing raids around 10:15 am (0815 GMT)," a Ras al-Ain activist who identified himself as Hevidar told AFP over the Internet.
"These led to the death of nine civilians, including a child, not more than eight years old... and many of the wounded civilians were transported to hospitals in Turkey," said Hevidar.
Many people who had recently returned to the town after being displaced during previous violence were forced to flee once again across the border, he added.
In unverified video footage posted on the Internet by activists, a group of men wearing coats are seen lifting pulling bloodied and limp bodies from beneath a pile of steel and other collapsed building materials.
A fire can be seen blazing in the background as the men load the bodies into the back of a truck.
Ras al-Ain was the scene of fierce clashes between fighters loyal to the Kurdish Democratic Union Party (PYD) -- which has close ties to Turkey's rebel Kurdistan Workers' Party (PKK) -- and the Al-Nusra Front and Ghuraba al-Sham in late November.
The Observatory, which relies on a network of activists, lawyers and medics, later reported that clashes had died down after the two parties managed to strike an agreement.