Nearly a week after seizing strategic Palmyra, ISIL militants gathered 20 men, who they said were fighting for the Syrian government, in the ruins of the theater and shot them dead, Rami Abdel Rahman, head of the Syrian Observatory for Human Rights, told AFP.
"ISIL gathered a lot of people there on purpose, to show their force on the ground," he said.
Syria's antiquities director said he feared the killings were a harbinger of the much-dreaded destruction of the ancient site, considered one of the world's greatest heritage jewels.
The ISIL has damaged priceless historical sites across the region but mainly used its sledgehammers and dynamite on statues and places of worship it considers idolatrous.
The terrorist group seized Palmyra on May 21, a move analysts warned positioned the group to launch more ambitious attacks on Damascus and third city Homs.
According to the Observatory, it has over the past week executed at least 217 people, including 67 civilians, in and around the city.
In neighboring Iraq, the government's efforts to pressure the ISIL in its Anbar stronghold gathered pace, with thousands of fighters deployed across the province from different directions.
Their immediate goal was to cut off the group's supply lines, but some forces inched toward provincial capital Ramadi which the insurgents captured on May 17.