ISIS militants have beheaded a Syrian antiquities expert in Palmyra on and hung his headless body from a column at a public square.

"IS executed Dr. Khaled al- As'ad, who has a doctorate in archaeology, in the public square in Palmyra in front of dozens of people where they slaughtered him by a knife after arresting him a month ago," the Syrian Observatory for Human Rights said in a statement

Unverified pictures posted online appeared to show a headless body hanging from a column at a public square beside a sign in Arabic accusing 82-year-old Asaad of supporting the regime, reported Middle East Eye.

Asaad, who had worked as an antiquities chief at Palmyra for more than 50 years before jihadists captured the ancient Syrian site in May this year, was arrested by the Islamic State group last month.

"Just imagine that such a scholar who gave such memorable services to the place and to history would be beheaded ... and his corpse still hanging from one of the ancient columns in the center of a square in Palmyra," Syrian state antiquities chief Maamoun Abdulkarim told Reuters.

"The continued presence of these criminals in this city is a curse and bad omen on (Palmyra) and every column and every archaeological piece in it," Abdulkarim said.

ISIS jihadists have already destroyed some parts of Palmyra's ancient ruins and threatened to completely destroyed UNESCO World Heritage site, according to DW. The terrorist group controls around one-third of both Syria and Iraq and established self-claimed Caliphate in Raqqa.