The jihadists have destroyed cultural treasures across the Middle East and North Africa, often describing the sites as idolatrous.
Although Isis was reported to have mined sections of Palmyra last month, the ancient ruins were thought to have been left undamaged in an attempt to curry favour with local residents as it consolidated power in a nearby town bearing the same name.
Yesterday, however, Isis posted photos of a civilian being forced to destroy what appeared to be priceless statues plucked from Palmyra's undulating ruins.
One image, apparently taken in the Syrian city of Aleppo, showed the jihadists flogging a resident as he smashed a statue for the gathered crowd. In others, the militants joined in the public destruction themselves. The accompanying text said the "contraband" artefacts were found in the possession of a lone smuggler.
Since sweeping to power across large swathes of Iraq and Syria last year, Isis has tried to establish a monopoly on the smuggling of artefacts. It has even established a Ministry of Antiquities to regulate the process.
The Telegraph has obtained Isis-stamped licences from branches in Aleppo and Deir Ezzor permitting residents to excavate archaeological material, apparently in return for a sum of money.
Involvement in the trade has reportedly raised the extremist group tens of millions of dollars, a similar sum to that raised through the kidnap and ransom of Western hostages.
Irina Bokova, the head of the United Nations cultural agency Unesco, said on Wednesday that one fifth of Iraq's estimated 10,000 official sites had been heavily looted under Isis control.
Some sites in Syria had been ransacked so badly they no longer had any value for historians and archaeologists, she said, describing the damage as "cultural cleansing".
Archaeologists monitoring developments in Isis-held areas said they began to see evidence of pillage on a level unseen throughout the three destructive years of Syria's civil war.
In Isis-controlled territory around the Mesopotamian city of Mari, a trade hub founded in 300 BC, more than 1300 excavation pits have been dug in the past few months, according to satellite imagery.
"The deliberate destruction, what we are seeing nowadays in Iraq and Syria, has reached unprecedented levels in contemporary history," Bokova told the Royal United Services Institute in London.
