BEIRUT—Islamic State fighters blew up ruins of one of the most venerated temples in the ancient city of Palmyra, according to antigovernment activists, the latest attack in the extremist group’s campaign to rid Syria of what it views as un-Islamic artifacts.

It wasn’t immediately clear how much of the 2,000-year-old Temple of Bel remained after Sunday’s demolition, said the Syrian Observatory for Human Rights, a U.K.-based activist and monitoring group.

The Roman-era edifice, surrounded by stately, 59-foot-columns, was one of the most important religious buildings of the first century A.D., according to Unesco, the United Nations’ cultural agency.

Massumeh Farhad, chief curator and curator of Islamic art at the Freer Gallery of Art in Washington, D.C., said the city of Palmyra—and the Temple of Bel specifically—have inspired architecture in the U.S. capital.

Earlier in August, Islamic State forces destroyed Palmyra’s Temple of Baalshamin, which also dates back almost 2,000 years.