Khaled Omran, an APSA member and former Palmyra resident now living in Turkey, said: “The archaeological area is under control of the Russians. They don’t even allow the Syrian regime troops and Palmyra’s residents to enter it without permission.”

The looters are smuggling the antiquities out of Syria through neighbouring Turkey and Lebanon, said Mr Omran, who closely monitors the trade. APSA said statues, tiles and coins – which records show had been looted in the last six months – being offered on the black market in Turkey.

In February 2015 the UN banned all trade in artefacts from the region. But experts say a large number of objects continue to find their way to Western buyers, who pay thousands of pounds for the Roman and Byzantine ruins.

Chris Doyle of the Council for Arab-British Understanding said: “The (Syrian) authorities will know this is going on. The likelihood is any further missing artefacts will just be blamed on Isil.

“The regime has no better record than Isil on protecting such heritage, as the pulverising of Aleppo demonstrates.”