Victims include 8 children in the eastern Syrian city of Ashara, Syrian Observatory for Human Rights says.
Syrian Observatory for Human Rights said air stikes thought to be carried out by Syrian and Russian forces killed at least 17 people, including 8 children in a market in the Syrian city of Ashara, near the southeast provincial capital Deir al-Zor city on Monday.
According to the British-based human rights organisation, it is feared the death toll will rise because of the high number of serious injuries.
The Syrian regime, which has been supported by Russia in aerial strikes across the country since September 2015, began attacking DAESH last week.
But the Russian Defence Ministry has denied its warplanes carried out air strikes on the market place.
Many civilians who are unable to move in and out of DAESH-held areas face the risk of being killed by constant air strikes.
UN says such offensives are successful in making gains on DAESH. They also put millions of people at risk of displacement.
The UN'S Office for the Coordination of Humanitarian Affairs said last week that 20,000 people have already been displaced as a result of the Kurdish-led offensive in Eastern Aleppo's Abu Qalqal and Menbij city sub-districts which are mostly under the control of DAESH.
According to Syrian government sources, the army captured crossroads to advance to Raqqa, in eastern Aleppo, and the strategic city Deir al Zour which connects the territory to Iraq.
The recent attack which killed civilians in the DAESH controlled territory came shortly after another strike by regime forces. At least 15 civilians were killed in air strikes on Friday in the DAESH-held area of Boulil near Deir al Zour.
More than 500 casualties in Aleppo
The Syrian Observatory for Human Rights also said on Monday that "hysterical bombing" continues in another neighbouring city of Aleppo, also on the forty-fifth consecutive day.
At least 502 civilian citizens have been killed and 2,600 civilians citizens injured in regime and Russian bombings that caused great destruction in public facilities and hospitals, said the Observatory. Among those killed were 105 children and 76 women over the age of eighteen.