A blast on a bus in central Damascus Sunday killed at least seven people and wounded 20 others, Agence France-Presse reported. The explosion in Syria’s capital city tore through a bus carrying Shiite Muslim pilgrims who were visiting religious sites, according to Syrian Observatory for Human Rights director Rami Abdel Rahman.

It was not immediately clear what caused the blast in the Souq al-Hamadiyeh neighborhood of Damascus, AFP said.

Syrian state media also reported on the blast, but with slightly different numbers. The official Syrian Arab News Agency said at least four people had been killed and 19 wounded.

The explosion occurred close to the medieval citadel in the city center, an area that has been rarely targeted during the four-year Syrian Civil War, as reported by the Associated Press.