The bombing was the latest in a series of attacks attributed to Iran that have targeted Israelis and Jews overseas and threatened to escalate a shadow war between the two arch-enemies. Iran has denied involvement in the past but did not comment on Wednesdayâ€™s attack.
President Barack Obama termed it a â€œbarbaric terrorist attackâ€ and called Israeli Prime Minister Benjamin Netanyahu to pledge U.S. help in finding the perpetrators.
The blast gutted the bus at the airport in the quiet Black Sea resort city of Burgas, some 400 kilometers (250 miles) east of the capital, Sofia, where the Israelis had just arrived on a charter flight from Tel Aviv carrying 154 people, including eight children.
Black smoke billowed into the sky from the stricken bus after the bomb exploded. Young Israelis said they were just boarding when the blast ripped through the white vehicle in the airport parking lot. Israeli Foreign Minister Avigdor Lieberman said at least seven people were killed.
â€œWe were at the entrance of the bus and in a few seconds we heard a huge boom,â€ said Gal Malka, an Israeli teenager who was slightly wounded.
The resort town has become a popular travel destination in recent years for Israelis, particularly for recent high school graduates before they are drafted for mandatory military service.
No group immediately claimed responsibility for the attack, which wounded 30 others. But suspicion immediately fell upon Iran and its Lebanese proxy, the Hezbollah guerrilla group.
â€œAll signs point to Iran,â€ Netanyahu said. â€œJust in the past few months, we have seen attempts by Iran to harm Israelis in Thailand, India, Georgia, Kenya, Cyprus and more. This is an Iranian terror attack that is spreading across the world. Israel will react forcefully to Iranâ€™s terror.â€
The Israeli leader gave no evidence to back his charges.