The Roman-era amphitheatre in Palmyra's World Heritage Site is now the venue for a macabre show.
The Islamic State is using the ancient Roman-era ruins dating back to 2000 years to carry out gory executions. The Isis fighters have already executed 20 people at the historic Roman theatre, the Syrian Observatory for Human Rights said.
Isis summoned locals to the theatre to watch the executions before shooting the men in their heads, the monitoring group said.
"IS executed 20 men by firing on them in front of a crowd gathered in Palmyra's Roman theatre, after accusing them of fighting for the Syrian regime,' Rami Abdel Rahman, head of the monitoring group, told AFP.
After taking over the ancient city of Palmyra in Syria last week, the Islamic State militants have reportedly killed more than 200 people so far.
The group also threatens to raze the ancient ruins that include Roman columns, amphitheatres and the Temple of Bel, which are classified as a World Heritage Site by UNESCO.
The group released its first video from Palmyra on Tuesday, in which black smoke was seen rising from behind the ancient ruins, raising concerns over the fate of the historic relics.
However, Isis has reportedly said that it will not harm the monuments and ruins, but will only "destroy statues", IBTimes UK reported.
An Isis commander reportedly made the declaration through a Syrian radio station.