Mass executions by ISIS in the Syrian-Christian town of Al-Qaryatayn have left more than 100 civilians dead, according to a monitoring group.

The militant group's killings over almost three weeks happened just days before the Syrian regime recaptured the town, according the UK-based Syrian Observatory for Human Rights, which has an extensive network of sources in Syria.

'ISIS has over a period of 20 days executed at least 116 civilians in reprisal killings, accusing them of collaboration with regime forces,' observatory chief Rami Abdel Rahman told the Agence France-Presse news agency.

The town, some 300 kilometres (190 miles) from another ISIS stronghold in Deir Ezzor, was recaptured by the militants in an unexpected attack earlier this month.

Over the weekend the Syrian government took the town back after more than 200 extremists withdrew overnight.

'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 Daesh 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.

Most of those killed were executed in the last two days before Daesh lost the town again, he added.

Despite the success in briefly retaking Al Qaryatayn, ISIS is largely on the retreat and last week its de facto capital in Raqqa was declared ISIS-free.

Before the Syrian civil war broke out in 2011, Al-Qaryatayn was considered a high-point for religious coexistence in the country with several hundred Christians among the 30,000 population.

But many churches were reduced to rubble during ISIS' eight-month long occupation.