Islamic State militants have blown up one of the most important temples in the ancient Syrian city of Palmyra, accelerating their relentless campaign of destruction against the historical treasures that have fallen under their control, activists and monitors said Sunday.

The 1st-century Temple of Bel lay at the heart of the complex of ruins that had made Palmyra one of Syria’s most significant archaeological and tourist attractions and earned it a designation as a United Nations Educational, Scientific and Cultural Organization world heritage site.

The Britain-based Syrian Observatory for Human Rights, which monitors events in Syria, and an activist said the militants had detonated explosives around the edifice, possibly destroying it.

Khaled al-Homsi, an activist who uses a pseudonym and is located outside Islamic State territory in Homs, where Palmyra is located, said the temple was blown up Sunday afternoon. He said the extent of the damage was unclear.

It was impossible to independently confirm the reports because of the lack of telephone connections and Internet service in Palmyra, which fell to advancing fighters of the Islamic State after Syrian government forces retreated in May.

Similar reports last week that the smaller nearby temple of Baal Shaamin had been destroyed turned out to be true. A few days later, the Islamic State published a series of photographs showing militants laying gas bottles and explosives around the temple and then seemingly detonating a huge explosion.

Satellite images of Palmyra released by the United Nations on Friday confirmed that the temple is no longer there.

The destruction of the Baal Shaamin site offered the first indication that the Palmyra ruins were on the militants’ target list, in contradiction of an earlier Islamic State statement that they were not.

In an audio recording released shortly after the Islamic State captured the town from government forces in May, an Islamic State fighter said the group had no reason to destroy the Palmyra site, because its goal is only to remove idolatrous depictions of human forms, not to attack buildings such as the stately columns and structures that comprise the Palmyra site.

The statement had raised the hopes of archaeologists and historians that Palmyra would be spared, perhaps as collateral against U.S. airstrikes targeting the town. The U.S.-led coalition fighting the Islamic State is not known to have conducted any airstrikes in the area, although the Syrian government reportedly has.

The Islamic State recognizes the value placed by its international foes on archaeological ruins and the propaganda value in destroying them, said Amr al-Azm, a history professor at Shawnee State University in Ohio, who is Syrian and is involved in efforts to preserve the country’s heritage from the ravages of the war.

Video posted by the United Nations Educational, Scientific and Cultural Organization in 2010 shows the temple of Baal Shamin in Palmyra, Syria, which Islamic State militants have reportedly destroyed. (UNESCO-NHK)

“It’s all part of a carefully crafted message that ISIS can act with impunity and the international community is incapable of stopping them,” he said, using an acronym for the Islamic State.

The Islamic State has wrecked numerous sites of priceless significance to the ancient heritage of the Middle East in Syria and Iraq, and it has often made a point of filming the process. Militants have smashed statues at Mosul Museum, razed the remains of cities in what was once ancient Nineveh, and destroyed churches and monasteries in Syria.

At the same time, the group is thought to have salvaged many smaller items to sell on the thriving black market for antiquities from the area.

Last week, the FBI urged U.S. antiquity dealers to be on the lookout for artifacts smuggled from Iraq and Syria, warning that purchasing items from the Islamic State could trigger charges related to terrorism.

Read more:

What Palmyra looked like in the 1800s and how the Islamic State is destroying it

Before and after images show the destruction of Palmyra’s treasured temple

The splendors of Palmyra, the ancient Syrian city that the Islamic State is destroying