Islamic State of Iraq and Syria (IS) militants executed 26 civilians on Thursday after the group overran villages near Syria's ancient city of Palmyra, the Syrian Observatory for Human Rights said. "IS executed 26 civilians, including at least 10 by beheading, after accusing them of collaborating with the Syrian regime," said Observatory head Rami Abdel Rahman, using the Arabic acronym for ISIS. Earlier on Thursday, the radical group battled Syrian troops near ancient Palmyra , threatening the UNESCO world heritage site with destruction like that which it has already wreaked in Iraq, the monitoring group said.