GETTY The Arch of Triumph was a major monument at the 2,000 year old site

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Rami Abdel Rahman, head of the British-based Syrian Observatory for Human Rights (SOHR), said ISIS "tied three individuals it had arrested from Palmyra and its outskirts to the columns...and executed them by blowing up" three columns. Khaled al-Homsi, a local activist from the besieged city, which is a Unesco World Heritage site, said ISIS had not yet informed local residents who the executed individuals were or why they had been killed.

He said: "There was no one there to see (the execution). The columns were destroyed and IS has prevented anyone from heading to the site." Video released last week showed the 2,000-year-old iconic Arch of Triumph being blown up by ISIS fighters.

GETTY Palmyra stands at the crossroads of several civilizations

GETTY The ancient site has been ravaged by ISIS

One pole of the arch is still standing, but most of the structure - which stands at the top of a colonnaded street that runs through Palmyra - has been completely destroyed.

Fears are growing that the terror group is preparing to blow up the Roman amphitheatre - which has been used as a venue for gruesome public murders. The jihadi group also beheaded Palmyra's 82-year-old former antiquities director in August.