The strikes hit the town of Ashara, about 60 km (40 miles) southeast of the provincial capital, Deir al-Zor city. Eight children were among those killed and the toll was expected to rise because of the high number of serious injuries, the Britain-based Observatory said, Reuters reports.
Warplanes bombed other areas of Deir al-Zor province, which is mostly under the control of Islamic State. The group has laid siege since March last year to the remaining government-held areas in Deir al-Zor city.
Syrian government forces launched an offensive against IS last week starting from Hama province further west, pressing toward the jihadists’ stronghold in Raqqa province, which borders Deir al-Zor.
Deir al-Zor province links Raqqa with Iraq, where the group also controls territory.
A Syrian military source said the army had captured a crossroads in its latest offensive from which it could advance toward Raqqa, Deir al-Zor or eastern Aleppo.