BEIRUT (AFP) - The ISIS (Islamic State in Iraq and Syria) terror group executed three people in Syria's ancient city of Palmyra by binding them to three historic columns and blowing them up, a monitoring group said Monday.

Rami Abdel Rahman, head of the Syrian Observatory for Human Rights, said IS on Sunday "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, an activist from Palmyra, said IS had yet to inform local residents who the executed individuals were or why they had been killed.

"There was no one there to see (the execution). The columns were destroyed and ISIS has prevented anyone from heading to the site," said Homsi, who works with the local Palmyra Coordination Committee activist group.

Mohammad al-Ayed, also an activist from Palmyra, said the columns were "archeological, and there are many like them still present in Palmyra".

"ISIS is doing this for the media attention, so that ISIS can say that it is the most villainous, and so it can get people's attention," al-Ayed said.

ISIS has captured swathes of territory across Iraq and Syria to create a self-styled "caliphate" where it enforces an extreme form of Islamic rule.

ISIS considers pre-Islamic artefacts to be idolatrous and therefore worthy of destruction.

Since the extremists seized Palmyra from regime forces in May, they have destroyed multiple sites and historic artefacts, including its celebrated temples of Bel and Baal Shamin as well as several funerary towers.

ISIS has used Palmyra's grand amphitheatre for a massacre in which child members of the group killed 25 Syrian soldiers, execution-style, in front of residents.

It also beheaded Palmyra's 82-year-old former antiquities director in August.

Palmyra's ruins are on the UNESCO World Heritage list, and before the war around 150,000 tourists a year visited the town.

Experts say the militants have used the destruction to raise their profile to attract new recruits, and are also funding their "caliphate" by selling treasures on the black market.

Syria's archaeology association, the APSA, says that more than 900 monuments and archeological sites have been looted, damaged or destroyed during the four-year civil war.