Staying true to its promise of destroying the “idols that the infidels used to worship,” the Islamic State group blew up at least two shrines in the ancient Syrian city of Palmyra, according to media reports Tuesday. The destruction of the shrines has renewed concerns over the fate of other priceless monuments, artifacts and statues in the city, which was captured by ISIS in May.

According to a report by Al Jazeera, the two destroyed shrines have been identified as the tomb of Mohammed Bin Ali -- a companion of the Prophet Muhammad's cousin, Imam Ali -- and a  500-year-old shrine dedicated to Sufi scholar Nizar Abu Bahaa Eddine. Images showing the destruction of the shrines are being circulated online by ISIS sympathizers.

“They [ISIS] consider these Islamic mausoleums to be against their beliefs, and they ban all visits to these sites,” Syria's antiquities director Maamoun Abdulkarim told Agence France-Presse (AFP) Tuesday, claiming that the Sunni militant group has so far destroyed at least 50 mausoleums dating back 100 to 200 years in regions under its control. Several tombstones at a cemetery for Palmyra residents were also reportedly blown up.

“All tombs with marble designs were destroyed. For them [ISIS], graves should not be visible,” Abdulkarim reportedly said. “The city is a hostage in their hands, the situation is dangerous.”

According to the Syrian Observatory for Human Rights, ISIS has rigged the ancient city, considered a Unesco World Heritage Site, with mines and improvised explosive devices in order to prevent the advance of government forces. In recent days, Syrian troops have advanced in territories west of Palmyra and are currently believed to be planning an offensive to recapture the city.

ISIS has a history of destroying statues and artifacts it believes to be idolatrous. In February, the group had released a video showing its members toppling 3,000-year-old statues in the Mosul museum in Iraq. It also showed the militants attacking these artifacts with power drills, sledgehammers and pickaxes.