Islamic State carry out 20 executions in Palmyra's ancient Roman theatre

The Islamic State (IS) group has used the Unesco-listed Roman theatre of Syria's ancient city of Palmyra as an execution ground for 20 men who were killed in front of a crowd, a monitor said.

Nearly a week after seizing strategic Palmyra, IS gathered the men they accused of fighting for the Syrian regime in the ruins of the theatre and shot them dead, said Rami Abdel Rahman, head of the Syrian Observatory for Human Rights.

"IS 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 jihadist group has damaged priceless historical sites across the region but mainly used its sledgehammers and dynamite on statues and places of worship it considers idolatrous.

IS seized Palmyra on May 21, a move analysts warned positioned the group to launch more ambitious attacks on Damascus and third city Homs.