A Syrian air force plane is thought to have attacked a petrol station near the north-eastern town of Raqqa, killing over 30 people according to rebels.
The opposition group said people were queuing for petrol when they were attacked.
Rebels recently captured the nearby border post of Tal al-Abyad, and pictures showed the rebel flag flying next to the Turkish flag at the crossing.
The Syrian army has been attempting to recapture the border town, with reports of severe clashes emerging.