BERLIN — A tour bus carrying beachbound Israeli vacationers exploded in a fireball on Wednesday outside an airport in the Bulgarian city of Burgas, killing at least six people and wounding dozens more in what Bulgaria, Israel and the United States called a terrorist bomb attack. Israel quickly blamed Iran and promised a firm response.

Photographs and video taken at the airport showed billowing black smoke from the explosion that left the bus a blackened skeleton, scorched several buses nearby, shattered windows and forced the airport to temporarily close. Witnesses quoted by Israeli news media said some victims were on fire as they tried to escape the bus. They said that many suffered severe burns and that body parts were flung around the parking lot.

Iran had no immediate comment on Israel’s accusations, and no group claimed responsibility for the blast, which the Bulgarian foreign minister said had been caused by a bomb placed in the bus’s luggage compartment. But if the Israeli accusations are confirmed, the Bulgarian blast would be the first successful attempt by Iranian operatives to kill Israelis in attacks abroad after a string of failed bomb plots targeting Israeli diplomats in Georgia, India and Thailand this year.

Even without such confirmation, the Bulgarian explosion escalated the tensions between Israel and Iran that are already high because of the Iranian nuclear energy program, which Israel has called a guise for Iran to develop nuclear weapons despite Tehran’s repeated denials.