Islamic State (IS) has killed three of its captives in Syria by tying them to Roman-era columns in Palmyra before blowing the structures up with explosives.

It was the latest gruesome method of killing by the militant group, which has become known for horrific beheadings of its prisoners.

A Palmyra activist said the killings of the three took place at the Palmyra archaeological site, located a few miles away from the city.

The Britain-based Syrian Observatory for Human Rights said the three were civilians but that their identities remain unknown.

IS has also destroyed many of ancient Palmyra's relics, including the Temple of Bel and the Arch of Triumph.