Islamic State militants killed three captives in Syria's ancient city of Palmyra by tying them to ancient temple columns and blowing them up, human right activists said on Tuesday.
The identities of those reportedly killed on Sunday have yet to be given.
But they are thought to be the first to have been killed in that manner since the jihadist group seized the ruins in May, Syrian Observatory for Human Rights reported.
IS has destroyed two 2,000-year-old temples, an arch and funerary towers at Palmyra, one of the most important cultural centres of the ancient world.
The group believes that such structures are idolatrous. The UN cultural agency, Unesco, has condemned the destruction as a war crime.
The Syrian Observatory for Human Rights is a Britain-based group that monitors the conflict in Syria.