Islamic State militants have blown up the Temple of Baal Shamin, one of the most important sites in Syria's ancient city of Palmyra, the country's antiquities chief Maamoun Abdul Karim says.

"Daesh placed a large quantity of explosives in the Temple of Baal Shamin today and then blew it up causing much damage to the temple," he said, using another name for the group.

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.

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 more than a month.

"We have said repeatedly the next phase would be one of terrorising people and when they have time they will begin destroying temples," Mr Abdul Karim said.

"I am seeing Palmyra being destroyed in front of my eyes. God help us in the days to come."

Before the UNESCO-listed city's capture by IS, Syrian officials said they moved hundreds of ancient statues to safe locations out of concern they would be destroyed.

In June, IS 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.

AFP/Reuters