Islamic State fighters have released at least 170 of 300 workers they had seized in an attack this week at a cement plant near Damascus but are expected to hold several dozen linked to the Syrian regime, a monitoring group said on Friday.
"I think they will release all the workers," Rami Abdel-Rahman, head of the Britain-based Syrian Observatory for Human Rights, told dpa by telephone.
The Observatory, which relies on a network of activists inside Syria, said that the 170 workers were released following mediation launched by local notables in an area near the town of al-Dumeir, where the cement factory is located.
The watchdog added in a statement that an unspecified number of the freed workers have arrived in the area north-east of Damascus.
"The group was still trying to contact sources in some areas to confirm more releases,” Abdel-Rahman said.
"They will keep between 30 and 40 security guards because they are working as labour for the [Syrian] regime," he added.
There was no official comment in Damascus.
Syrian state television reported the abduction of more than 300 workers on Thursday, citing an unnamed official at Syria's Industry Ministry.
The kidnappings had reportedly been confirmed by managers of the factory.
Islamic State has this week mounted a spate of attacks in government-ruled areas near Damascus.
The attacks are seen as retaliation for the military setbacks suffered by the al-Qaeda splinter group elsewhere in Syria.
Last month, Syrian regime forces, backed by Russian warplanes, drove Islamic State from the ancient city of Palmyra, which the militants had controlled for 10 months.
On Sunday, Syrian forces recaptured the town of al-Qaraytain, about 100 kilometres south-west of Palmyra, from Islamic State.