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The bodies of at least 67 Syrian civilians, many summarily killed by the Islamic State group, have been discovered in a central town in Syria retaken from IS by government troops over the weekend, the Syrian government and activists said Monday.

A senior Syrian official described the attack as a "shocking massacre," saying the search and documentation of those killed in the town of Qaryatayn, in Homs province, is still under way.

The news of the gruesome find began to emerge first late on Sunday. The number of bodies was likely to climb.

Some were shot in the street as IS militants retreated from the town, gunned down because they were suspected of working with the governments, according to activists. At least 35 of the casualties were found shot and their bodies dumped in a shaft.

The apparent revenge killings underscore the Islamic State group's ability to inflict heavy losses in Syria even while its militants are on the retreat in north and eastern Syria, days after having been defeated in Raqqa, the group's one-time "capital" of its self-proclaimed caliphate. They also raise the spectre of more revenge killings by the group while it fights to hang on to its last strongholds in Syria.

An Associated Press video, filmed as Syrian government troops recaptured Qaryatayn, showed several bodies in the streets of the town. In the video, a town resident says IS "monsters" killed more than a 100 people, including soldiers and civilians.

"These are people who don't know God, they don't know anything. They killed children and women with knives, they beat women, broke their arms," he said, speaking on condition of anonymity fearing for his own safety.

Talal Barazi, the governor of Homs province, told The Associated Press on Monday that most of the bodies were of townspeople who were government employees or were affiliated with Syria's ruling Baath party.

He said the killings went on for the three weeks that IS was in town and "terrorized" its residents, adding that at least 13 residents remain missing while six bodies have not been identified.