Islamic State has blown up part of the Temple of Bel - a Roman-era structure in the center of the desert city of Palmyra.
Last week, Islamic State detonated explosives in the Baal Shamin temple.
The extent of the damage to the World Heritage site isn't known.
Islamic State seized Palmyra from government forces in May and has kept a tight grip on communications in the city.
This month the group beheaded the 82-year-old Syrian archaeologist who had looked after Palmyra's ruins for four decades. Militants hung his body in public, according to Syria's antiquities chief.
Islamic State has destroyed a number of historic sites and monuments across Iraq and Syria.