The Islamic State of Iraq and the Levant (ISIL) have shot dead at least 20 men in the ruins of Syria's ancient city of Palmyra, accusing them of fighting for the government, an activist monitoring group has said.

"IS[IL] 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 UK-based Syrian Observatory for Human Rights, told the AFP news agency.

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

ISIL have carried out a string of atrocities, including videotaped beheadings and mass killings, rape and enslavement in areas it controls in Syria and neighbouring Iraq.

The killings, 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 theatre to execute people proves that these people are against humanity," he added.

ISIL seized Palmyra, known as Tadmur in Arabic, including its UNESCO world heritage site, on May 21, after a bloody assault that lasted nine days.

The UK-based observatory said ISIL 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 are at "risk to be exposed to arbitrary violent actions and more destructions of cultural sites might be perpetrated," Mogherini said.

ISIL "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.

Source: AFP