ISIS has executed three captives by binding them to ancient columns in Palmyra and blowing them up, a monitoring group has said.

The killings, which appear to be ISIS' latest method of execution, happened on Monday afternoon at the Palmyra archaeological site a few miles from the city.

ISIS "tied three individuals it had arrested from Palmyra and its outskirts to the columns... and executed them by them blowing up," said Rami Abdel Rahman, the head of the Syrian Observatory for Human Rights.

Local residents are yet to be informed who the individuals were and why they were executed, said Khaled al-Homsi, an activist from Palmyra.

"There was no one there to see [the execution]," he said. "The columns were destroyed and ISIS has prevented anyone from heading to the site."

Palmyra was captured by ISIS in May and the militant group has since destroyed many of the city's ancient artefacts and historical sites, including the temples of Bel and Baal Shamin as well as several funerary towers, according to the MailOnline.

The columns blown up in Monday's execution were "archeological, and there are many like them still present in Palmyra," said Mohammad al-Ayed, another activist from Palymyra.

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

Islamic State has reportedly also used the ancient grand amphitheatre in Palymra as an exucution arena. Twenty-five Syrian soldiers were recently shot there by child members of the militant group.

This week's execution comes shortly after ISIS killed a 19-year-old Syrian by running him over with a tank.