Wednesday’s killings brought to 237 the number of people executed by IS since May 16 in the course of an offensive in the eastern part of the central Syrian province of Homs, according to data from the observatory, which is based in Britain.

That figure includes 67 civilians – 14 of them children – who were accused of collaborating with the regime and hiding pro-government fighters in their homes.

Syrian authorities said IS has beheaded at least 400 people, mostly women, children and the elderly, in Palmyra since seizing control of the city on May 20.

Palmyra, an oasis in the Syrian desert, was a leading cultural center in the 1st and 2nd centuries A.D. and later became an important stop for Silk Road caravans.

Prior to the start of Syria’s civil war, in March 2011, the Palmyra ruins were one of the country’s foremost tourist attractions. The UN Educational, Scientific and Cultural Organization has designated the ruins as a World Heritage Site.