The Islamic State has blown up three ancient funeral towers in the city of Palmyra, Syria’s antiquities chief, Maamoun Abdulkarim, said Friday, continuing the destruction of a World Heritage site that Unesco has condemned as a war crime. The militants, who had already destroyed two Roman-era temples in the city, blew up the tombs, which date from between A.D. 44 and 103, Mr. Abdulkarim said. The towers were sandstone constructions built to hold the remains of the old city’s richest families. Islamic State fighters, who seized Palmyra from the control of the Syrian government in May, had blown up the Temple of Baal and the Temple of Baalshamin in recent weeks, according to a United Nations analysis of satellite images.