ISIS has reportedly slaughtered hundreds of civilians after seizing control of the ancient town of Palmyra last week.
Syrian state TV reported that over 400 civilians had been massacred by the group since Wednesday of last week, while an additional 300 of President Bashar al-Assad's troops and loyalists, were hunted down and executed after the fighting died down.
"Shabiha [a term used to describe pro-regime militias and supporters], including men and women, were the ones targeted and killed," said Nasser, of the Palmyra Media Centre
Nasser also confirmed that besieged Palmyra, now under ISIS control, is not allowing civilians to leave or enter it, and that basic services have been cut off in the town.
"Water, electricity and phone landlines have been cut off there," he added.
Meanwhile, the Syrian military was deploying troops near Palmyra, in preparation for a counterattack to retake it.
Local activists reported that government fighter jets carried out more than 10 attacks on the city on Monday morning.
Talal Barazi, the governor of the central province of Homs, which includes Palmyra, told the Associated Press news agency on Sunday that there were plans from the government forces to launch a counterattack against ISIS fighters in Palmyra.
"There are plans, but we don't know when the zero hour for a military act in Palmyra [will be]," Barazi said.
The capture of Palmyra raised the prospect of ISIS destroying one of the world's most spectacular archaeological sites - a well-preserved, 2,000-year-old Roman city on the town's edge - just as they destroyed others in Syria and Iraq.