JERUSALEM — Israeli officials blamed Iran and vowed to retaliate for an explosion Wednesday aboard a bus carrying young Israeli tourists in Bulgaria that killed at least seven and wounded many others.

Israeli Prime Minister Benjamin Netanyahu called the explosion "an Iranian terror attack," adding: "Israel will react strongly to Iran's terror."

President Obama termed it a "barbaric terrorist attack." He called Netanyahu to express his sorrow over the attack and pledged assistance to bring the perpetrators to justice, according to statements from the two leaders' offices.

Israeli officials didn't say how they'd concluded that Iran was responsible for the blast, which struck the bus as it was heading to a parking lot at the international airport in Burgas, Bulgaria, a popular destination for Israelis. The Black Sea resort town 250 miles east of the capital, Sofia, has become a popular destination, 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.

Guy Metel, an Israeli interviewed by Israel's Channel Two news, said he was part of a group that had arrived on a trip organized by a Bulgarian tour company.

"We had just come out of the airport, and me and my family were told to get on bus number three," Metel said. "We had just done that when we heard a huge explosion and everything shook. One of the other buses was just this fiery burning shell and people were screaming."

He said that after removing his children from the bus, he tried to return to the scene of the bombing.

"It took forever for the paramedics to arrive. We were waiting and waiting, and I was holding a woman's head who was bleeding badly. I could hear others calling for help," he said.

Officials in Bulgaria said a suitcase filled with explosives had been placed on the bus and detonated remotely.

Wednesday's bombing coincided with the 18th anniversary of the bombing of a Jewish community center in Buenos Aires, Argentina, that killed 85 people. That attack also was blamed on Iran.

Iran has accused Israel of being behind a series of attacks on Iranian nuclear targets, ranging from the assassinations of Iranian nuclear scientists to mysterious computer viruses that have damaged Iranian centrifuges.

Israel has never admitted to involvement, but it and others have accused Iran of reprisal missions, including a February bombing in New Delhi that wounded an Israeli diplomat's wife and the discovery of a cache of explosives in Bangkok that Thai officials claim was linked to a plot to target Israeli diplomats.

Material from The Associated Press is included in this report.