A Syrian opposition monitoring group and a news agency linked to the Islamic State group say scores of cement works abducted near Damascus this week have been released.

Saturday's reports came two days after IS abducted 300 cement workers and contractors from their workplace in the areas of Dumeir just northeast of the capital Damascus.

The Britain-based Syrian Observatory for Human Rights said all those abducted have been released except for 30 people who were guards at the cement factory.

The IS-affiliated Aamaq news agency said most of the 300 were released after they were questioned to check whether they were all Muslims or included government supporters.

It said four workers who belonged to the minority Druse sect were killed and 20 other pro-government gunmen are still being held.