"ISIS has over a period of 20 days executed at least 116 civilians in reprisal killings, accusing them of collaboration with regime forces," said Rami Abdel Rahman, head of the Britain-based Syrian Observatory for Human Rights monitor.
Regime forces retook Al-Qaryatayn, which lies in the central Homs province, on Saturday, three weeks after the jihadists seized control of it.
ISIS had first occupied the town in 2015 and lost it to a Russian-backed Syria forces last year.
"After the regime retook it (on Saturday), the town's residents found the bodies on the streets. They had been shot dead or executed with knives," Abdel Rahman said.
"Most of the ISIS fighters who attacked the town a month ago were sleeper cells. They are from the town, know the town's residents and who is for or against the regime," he said.
The majority of those killed were executed in the last two days before ISIS lost the town again, he added.
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