At least three passengers were killed in an explosion on a tour bus carrying Israeli tourists in Bulgaria in what has been called a terrorist attack.

Israeli and Bulgarian reports had as many as seven people dying in the explosion Wednesday afternoon at Sarafovo Airport in Burgas; 20 were injured. The explosion hit one of three tour buses carrying Israelis, Israel’s Channel 1 reported.

The explosion occurred while the bus was still in the terminal. Burgas, located on the Black Sea Coast, is a popular tourist spot for Israelis.

Channel 1 quoted Bulgarian Foreign Minister Nikolay Mladenov as calling the explosion a terrorist attack.