Syrian mediators reached an agreement Friday for the release of some 300 cement workers kidnapped by the Islamic State group, a monitor said, but it was unclear exactly how many were freed. 

IS abducted the employees on Monday from Al-Badia cement factory outside the town of Dmeir, around 50 kilometres (30 miles) east of Damascus. 

Local figures from Dmeir mediated a deal with IS on Friday to let the workers go free, according to the Syrian Observatory for Human Rights. 

The Britain-based monitor said that in effect, some 170 workers would be freed as others had already managed to escape.

The Observatory could not specify how many had been released on Friday.

But a military source told AFP that he saw dozens of cement workers arrive on Friday evening at a nearby regime-held military airport. 

IS reportedly abducted the employees during a major offensive on Monday around the town of Dmeir.

The IS-affiliated Amaq news agency said in an online statement that it released about 300 of the workers, but that it would not free 20 men accused of belonging to a pro-government militia. 

The statement said four of the cement workers were executed for being Druze, an offshoot of Islam considered heretical by IS.

Dmeir is divided between IS control in the east and rebel control in the west, but several key positions around it, including the military air base and a power plant, remain in government hands.

On Friday, warplanes carried out fresh strikes around the Dmeir military airport, the Observatory said.

IS has carried out mass kidnappings in previous offensives.

In January, it abducted more than 400 civilians, including women and children, as it overran parts of Deir Ezzor province in the east. 

In northeast Syria last year, IS kidnapped at least 220 Assyrian Christians, many of whom have since been released through local negotiations.