Islamic State (IS) militants have kidnapped at least 400 civilians in the eastern Syrian city of Deir al-Zor according to the Syrian Observatory Human Rights.

Families of pro-government fighters were reportedly among those abducted, after government-held areas in the city were attacked on Saturday.

The monitoring group also reports that airstrikes on Islamic State's de facto capital city of Raqqa in Syria yesterday killed at least 40 civilians - but said it was unclear whether the airstrikes were carried out by the US-led coalition or Russian aircraft.

Of the civilians captured by IS, the Observatory's head Rami Abdulrahamn said: "There is genuine fear for their lives, there is a fear that the group might execute them as it has done before in other areas."

Deir al-Zor is the main town in a province linking Raqqa with territory controlled by the terror group in neighbouring Iraq.

Syria's state news agency SANA reported at least 300 people, including women and children, had been killed during the attacks in Deir al-Zor.

A source close to the Syrian government side said on Saturday that some of those killed had been beheaded. The reports have not been independently verified.

The Observatory said it was able to document the killing or execution of at least 135 persons and members since the attacks on Saturday in Deir al-Zor.