The Islamic State jihadist group shot dead at least 20 men in the ruins of Syria's ancient city Palmyra on Wednesday, accusing them of fighting for the government, a monitor said.

"ISIS 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 Syrian Observatory for Human Rights, told AFP.

"ISIS gathered a lot of people there on purpose, to show their force on the ground," Abdel Rahman said.

ISIS has carried out a string of atrocities including videotaped beheadings and mass killings, rape and enslavement in areas it controls in Syria and neighboring Iraq.

The executions, if confirmed, could signal the start of "the group's barbarism and savagery against the ancient monuments of Palmyra," Syrian antiquities director Mamoun Abdulkarim told AFP.

"Using the Roman theater to execute people proves that these people are against humanity," he added.

ISIS seized Palmyra, including its UNESCO world heritage site, on May 21, after a bloody assault that lasted nine days.

The Britain-based Observatory said ISIS had since executed at least 217 people, including 67 civilians, in and around the city.

EU foreign policy chief Federica Mogherini warned last Thursday of potential war crimes in Palmyra.

Thousands of people "risk to be exposed to arbitrary violent actions and more destructions of cultural sites might be perpetrated," Mogherini said.

ISIS "mass killings and deliberate destruction of archaeological and cultural heritage in Syria and Iraq amount to a war crime according to the Rome Statute of the International Criminal Court," she said.