Islamic State militants on Sunday blew up the temple of Baal Shamin, one of the most important sites in Syria's ancient city of Palmyra, said Maamoun Abdul Karim, the country's antiquities chief.
The temple bombing would be the first time the insurgents, who control swathes of Syria and Iraq and captured Palmyra in May, damaged monumental Roman-era ruins.
"We have said repeatedly the next phase would be one of terrorizing people and when they have time they will begin destroying temples," Abdul Karim told Reuters.
"I am seeing Palmyra being destroyed in front of my eyes," he added. "God help us in the days to come."
A week ago, the militants beheaded Khaled Asaad, an 82-year-old scholar who worked for more than 50 years as head of antiquities in Palmyra, after detaining and interrogating him for over a month.
Before the city's capture by Islamic State, Syrian officials said they moved hundreds of ancient statues to safe locations out of concern that the militants would destroy them.
In June, Islamic State blew up two ancient shrines in Palmyra that were not part of its Roman-era structures but which the militants regarded as pagan and sacrilegious.
The militants were also beginning excavation for gold and giving licenses for illicit excavation of the city's treasures, Abdul Karim added.