BEIRUT—Islamic State militants have destroyed two historic mausoleums in Palmyra, Syria’s top antiquities official said Wednesday, raising fears the extremists could next target the town’s famed Roman ruins.

Maamoun Abdulkarim, the head of the government’s Antiquities and Museums Department, told the Associated Press that the extremists destroyed the grave of Mohammad Bin Ali, a descendant of Imam Ali, cousin of Islam’s Prophet Muhammad and a deeply revered Shiite saint. The grave was just north of Palmyra.

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