Most of abducted cement workers in Syria set free

However, at least 30 are still held by IS, the Britain-based Syrian Observatory for Human Rights said, adding that their fate remains unknown.

At least 100 of those released were freed after questioning to determine their religion and whether they support the government, IS-affiliated Aamaq news agency said.

Four of the cement workers were killed for belonging to the minority Druze sect, an off-shoot of Shia Islam which the militant group considers as heretics deserving death.

At least 20 pro-government workers were still being held by IS.

Relatives of those still in detention spoke of their concern.

"I heard that IS executed some Druze. My relative [there] is Druze and I'm scared that he's one of them. My mother is beside herself with worry and we're very afraid of receiving bad news," a Damascus resident told AFP.

Reports of the abduction had emerged two days after IS took hostage the cement workers and contractors from their workplace in Dumeir, just northeast of the capital, after a surprise attack on government forces.

Initial reports claimed at least 175 of the cement workers were massacred by IS.

Most of those released had returned to their homes late Friday.

IS had 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.

Agencies contributed to this report.