More than 30 people were wounded in the attack against tourists headed for Bulgarian Black Sea resorts that also left two nearby buses in flames.
Witnesses described how panicked passengers jumped from bus windows and bodies lay strewn on the ground with their clothes torn off as ambulance sirens wailed and black smoke rose over Sarafovo airport in Burgas.
Israeli Prime Minister Benjamin Netanyahu, whose government said it had been told the death toll was seven, said “all signs point to Iran” after an official in Jerusalem said the bus was shot at and an explosive thrown.
“Israel will respond forcefully to Iranian terror,” Netanyahu said. “In the past few months we have seen attempts by Iran to harm Israelis in Thailand, India, Georgia, Kenya, Cyprus and other places.”
“The vast majority of the dead and the injured are Israelis,” Israeli foreign ministry’s deputy spokesman Paul Hirschson said.
The attack – the deadliest on Israelis abroad since 2004, and the first on Bulgarian soil against Israelis – drew strong condemnation from its staunchest ally the US, as well as from France and Britain.
US President Barack Obama condemned the “barbaric terrorist attack”.
Obama did not mention Iran in his written statement but reaffirmed America’s “unshakeable commitment to Israel’s security, and our deep friendship and solidarity with the Israeli people.”
“My thoughts and prayers are with the families of those killed and injured, and with the people of Israel, Bulgaria, and any other nation whose citizens were harmed in this awful event,” he said.
“These attacks against innocent civilians, including children, are completely outrageous. The United States will stand with our allies, and provide whatever assistance is necessary to identify and bring to justice the perpetrators of this attack.”
The attack came on the 18th anniversary of an attack on a Jewish community centre in Buenos Aires in 1994 that killed 85 people and wounded 300.
Israel also held Iran responsible back in 1994. The Islamic republic denied the charge and has accused Israel of being behind the assassinations of nuclear scientists and senior military figures in recent years.
“Six people were killed and over 30 were injured in a blast on a tourist bus near Sarafovo airport,” the Bulgarian foreign ministry said, adding that authorities were investigating whether it was a terrorist attack.
Israel’s foreign ministry said that Bulgarian Foreign Minister Nikolay Mladenov had told his counterpart Avigdor Lieberman that “the explosion was caused by a bomb,” while putting the death toll at seven.
Burgas mayor Dimitar Nikolov, who was at the airport at the time, said the blast happened as tourists were boarding the bus and placing their bags in the luggage compartment, where he suspects the explosives might have been.
“I cannot say if it was an act of terrorism. The blast was very strong and in my view was triggered by a device which was deliberately set,” he said.
The blast occurred around 1400 GMT on the bus carrying Israelis who had flown in to Burgas, the second largest city on Bulgaria’s Black Sea coast, setting off a fire that spread to another two buses, the ministry said.
Israel’s foreign ministry said the attack targeted a group of tourists who had just arrived on a flight from Tel Aviv.
Israeli media reports said many of those on the flight were young Israeli school-leavers who were just about to be drafted into the military.
Pictures showed huge plumes of black smoke rising over the airport, which the ministry said was closed after the attack.
Television showed ambulances rushing people to hospital and women crying.
Israel and formerly communist Bulgaria enjoy good relations, and the Black Sea coast has become a popular holiday spot for Israelis, with almost 140,000 visiting the country in 2011.
“I was on the bus and we had just sat down when after a few seconds we heard a really loud explosion,” one Israeli tourist, Gal Malka, told Israeli army radio. “The whole bus went up in flames.”
Aviva, another Israeli woman who was on a nearby bus, said she heard a “very loud explosion”.
“There are seven dead people,” she told the radio, adding that she saw people whose clothes had been blown off and bodies lying on the floor.
“It was just terrible; people were jumping out of the windows,” she said.
In January, Israeli public television reported that Bulgarian authorities had foiled a bomb attack when they found an explosive device on a bus chartered to take Israeli tourists to a ski resort.
Bulgarian troops were deployed in several ski resorts frequented by Israeli tourists after the bomb was found, the report added.
The Israeli military said it was about to send a plane carrying a medical team to Bulgaria “in order to provide medical care and to assist those injured in returning to Israel,” a statement said.