A senior ISIS spokesman and strategist known for his fiery sermons calling for attacks against the U.S. was killed in Syria, the group said Tuesday.

Abu Muhammad al-Adnani, one of Islamic State’s longest-serving and most prominent leaders, has been killed in Syria, the group’s Amaq news agency reported.

A founding member of the Islamic State, Mr. Adnani, a 39-year-old Syrian, was the group’s chief spokesman and propagandist, running an operation that put out slickly produced videos of beheadings and massacres that shocked the world and sent a rush of recruits running to join the group in Syria.

In Washington, the Pentagon spokesman, Peter Cook, confirmed that an American “precision strike” near Al Bab, Syria, on Tuesday night had targeted Mr. Adnani, but could not confirm his death. “We are still assessing the results of the strike,” he said in a statement.