Islamic State militants have murdered three people by strapping them to columns in the ancient city of Palmyra and blowing them, according to a respected monitoring group, Daily Express reports.

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.

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 amphitheater - 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.