Militants from the Islamic State group have demolished three ancient tower tombs in the Syrian desert city of Palmyra, the country’s antiquities chief said Friday.

Maamoun Abdulkarim told reporters that sources in Palmyra had confirmed the destruction of three tombs built between the years 44 and 103 A.D. "They blew up three tower tombs, the best preserved and most beautiful," he told Agence France-Presse.

ISIS militants recently destroyed two temples at the World Heritage Site, which it has controlled since wresting control of the city from the Syrian government in May.

The group also beheaded an archaeologist working at the site. In August, reports emerged that the group had beheaded Khaled Asaad, an antiquities scholar who worked at Palmyra.

In March, ISIS released videos showing the destruction of ancient sites in Hatra and Nimrud. The group has a history of ransacking and destroying art and artifacts deemed to be idolatry. ISIS posted videos of its militants demolishing 3,000-year-old statues in the Mosul museum in Iraq in February.

The United Nations has called the destruction of cultural heritage that ISIS is undertaking in Iraq and Syria a “war crime.”

The group had issued a statement in May promising that the iconic colonnades and buildings at Palmyra would be left untouched, and that the group would only target statues and shrines at the site.