He said there was an explosion Sunday inside the walls of the Temple of Bel, and while the extent of the damage is not yet known, witnesses report the walls are still standing. He called the site "the most important temple in Syria and of the most important in the whole Middle East."

For nearly 2,000 years -- back to the days when Christ walked the earth -- the Temple of Bel has been the center of religious life in the Syrian city of Palmyra.

But now, at least part of the most historically significant temple in Palmyra has been destroyed by ISIS, according to the UK-based Syrian Observatory for Human Rights, which relies on a network of activists in Syria for information.

ISIS has become known not only for its brutal executions, but for its hatred of antiquities -- and its wanton destruction of them.

Recently, it executed Khaled al-As'ad, an 82-year-old man who had spent his life on the painstaking task of preserving antiquities in Palmyra, because he refused to reveal where various irreplaceable relics had been hidden.

And now, apparently, ISIS has damaged the Temple of Bel.

Abdulkarim had told CNN on Monday that officials were working to confirm the reports with sources in the city.

'Meeting point' between classical, Eastern architecture

"We are waiting for details on the truth of what occurred, the exact location inside the temple, and the size of the destruction," Abdulkarim said.

The first-century temple, which is dedicated to the ancient "god of gods," is one of the largest and best-preserved in the region and represents a meeting point between classical and Eastern architecture, Abdulkarim said.

ISIS, perhaps the most brutal terrorist group to emerge in modern times, has shown a taste for demolishing irreplaceable ancient sites and antiquities. It considers "pre-Islamic religious objects or structures sacrilegious," wrote Sturt Manning, chairman of Cornell University's Department of Classics, in an opinion piece for CNN.com.

"It seeks to destroy diversity and enforce narrow uniformity. Evidence of a tolerant, diverse past is anathema," he said. "What it fears is memory and knowledge, which it cannot destroy."

Last week, ISIS published photos of its destruction of the temple of Baal Shamin, the first major structure in the ancient city of Palmyra to be destroyed.