A deadly explosion on a bus carrying Israeli tourists in Bulgaria is "clearly a terrorist attack," Israeli Defense Minister Ehud Barak said Wednesday.

"We have several people killed and dozens of wounded people," Barak said, as cited by CNN.

He said the attack was probably initiated by a group under the auspices of "either Iran or other radical Muslim groups," and named Hezbollah and Hamas as likely suspects.

“We have been tracking the intentions of terrorist groups such as Hezbollah, Hamas, Iranian groups and the Islamic Jihad to carry out attacks across the world for quite some time.

"We have a long struggle with them, which includes many successes, as well as difficult days. Today is one of those difficult days. It is important that Israelis continue to travel across the world, continue to travel in Israel and live their normal lives despite all the pain,” Barak said.