The Islamic State group killed at least 116 people in the Syrian town of Al-Qaryatain over a three-week period, according to the British-based Syrian Observatory for Human Rights.
The Syrian army recaptured the town in central Homs on Saturday.
"IS has over a period of 20 days executed more than 116 civilians in reprisal killings, accusing them of collaboration with regime forces," said Rami Abdel Rahman, head of the Britain-based monitor group.
Government officials and state media have not commented on the report.
Syrian troops and allied forces regained control of Al-Qaryatain "after eliminating terrorist Daesh [another term for IS] groups that had infiltrated it," state media said.
Earlier this month, the Observatory said IS fighters had seized the town from government forces, part of a counter-attack by the jihadists who have come under intense pressure in eastern Syria.
The town lies nearly 300 km from Deir al-Zor city in the east, the current focus of the government offensive, with Russian jets and Iran-backed militias, against IS.
Meanwhile, Russia has accused the US-led coalition in Syria of wiping the city of Raqqa "off the face of the earth" with carpet bombing.
The Russian Defence Ministry compared the bombing campaign to that waged on the German city of Dresden by US and British forces during World War II.
The ministry said it looked like the West was now rushing to provide financial aid to Raqqa to cover up evidence of its own crimes.
Major-General Igor Konashenkov, a spokesman for the Defence Ministry, said in a statement that around 200,000 people had lived in Raqqa before the conflict in Syria, but that not more than 45,000 people remained.
US-backed militias declared victory over IS in Raqqa, the group's capital, last week, raising flags over the last jihadist footholds after a four-month battle.
"Raqqa has inherited the fate of Dresden in 1945, wiped off the face of the earth by Anglo-American bombardments," Mr Konashenkov said.
Most of the German city was destroyed in Allied bombing raids just before the end of the war.
