IS has in recent months claimed responsibility for deadly attacks in Brussels and Paris, but has lost ground in Syria and Iraq.
Days after Syrian troops backed by Russian forces recaptured Palmyra and its ancient ruins, the army “uncovered a mass grave of officers, soldiers, members of the popular committees (pro-regime militia) and their relatives,” a military source told AFP on Saturday.
Twenty-four of the victims were civilians, including three children, he said, asking not to be named.
“They were executed either by beheading or by shooting.”
In a major symbolic and strategic coup for President Bashar al-Assad, the Syrian army last Sunday recaptured Palmyra and its UNESCO World Heritage Site, which IS had overrun in May 2015.
During their nearly 10-month occupation of Palmyra, the jihadists executed at least 280 people, according to the Syrian Observatory for Human Rights, a Britain-based monitor which confirmed the discovery of the mass grave.
Soon after IS stormed Palmyra, it shot dead 25 soldiers in the ancient Roman theatre.
It later released a video of the mass killing in which the executioners appeared to be children or teenagers.
Syria’s five-year war has left at least 270,000 people dead. Few mass graves have been found, however.
Nearly a week on, few of Palmyra’s up to 70,000 residents have returned.