Israel vowed to strike back at Iran for a brazen daylight bombing Wednesday that killed at least seven people on a bus full of Israeli tourists in Bulgaria.

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 Wednesday's attack.

U.S. President Barack Obama termed it a "barbaric terrorist attack" 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 kilometres east of the capital, Sofia, where the Israelis had just arrived on a charter flight from Tel Aviv carrying 154 people, including eight children.

Smoke rises into the sky after a bus exploded at Burgas airport, outside the Black Sea city of Burgas, Bulgaria, Wednesday. Witnesses told Israeli media that the huge blast occurred soon after someone boarded the vehicle. (Burgasinfo/Associated Press)

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.

"We were at the entrance of the bus and in a few seconds we heard a huge boom," 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.

'All signs point to Iran'

"All signs point to Iran," Netanyahu said. "Just 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 Iran's terror."

The Israeli leader gave no evidence to back his charges.

Top Israeli security officials were holding consultations late Wednesday. The Israelis said they were still weighing their options on how to respond, and there were no preparations under way for an immediate reaction.

The officials, speaking on condition of anonymity because they were not authorized to discuss a sensitive security matter, said any reaction would probably be a pinpoint operation limited in scope, most likely under the auspices of the Mossad spy agency.

The officials said security officials also were concerned about further attacks, similar to a string of incidents in India, Georgia and Thailand earlier this year, and were reviewing security at potential Israeli targets, such as airline terminals and diplomatic installations.

Bulgarian medics wheel a wounded woman into an ambulance after a deadly explosion at Burgas airport on Wednesday. (AFP/GettyImages)

The Bulgaria attack came exactly 18 years after the bombing of a Jewish community centre in Argentina that killed 85 people. An Argentine investigation concluded Iran was behind that attack. In addition, Israel has accused Iran of being behind a string of attacks in Europe, Asia and Africa in recent months.

The violence also came against the broader backdrop of the international standoff with Iran over its nuclear program. Israel, accusing Iran of developing nuclear weapons, has repeatedly hinted it is prepared to strike Iranian nuclear targets if Tehran does not curb its suspect program.

Iran accuses Israel of covert attacks

In the past, Iran has accused Israel of being behind a series of covert 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 against Israeli diplomats.

Israeli officials also have long warned that the Iranian-backed Hezbollah would try to attack Israelis abroad. Hezbollah has accused Israel of assassinating a top leader in Damascus in 2008 and vowed to avenge the killing. Israel has never admitted involvement in the mysterious explosion.

In a statement, Israeli Defence Minister Ehud Barak cited Iranian-backed militants and other radical Islamic groups as the likely perpetrators of the Bulgaria bombing.

"We are in a continual fight against them. We are determined to identify who sent them, who perpetrated [the attack], and to settle the account," he said.

Israeli counterterrorism expert Boaz Ganor said Wednesday's attack was "likely not the last in a series," adding, "All this looks like Hezbollah, Iran or a combination of the two."

Despite repeated alerts and concerns of an Iranian-backed attack in recent months, Israel said it had no advance intelligence on a pending attack in Bulgaria.

The attack took place near the airport, shortly after a charter flight filled with Israeli youth landed at 4:45 p.m. local time.

Bomb planted on bus

Lieberman, the Israeli foreign minister, said he was briefed by his Bulgarian counterpart and informed that a bomb was planted in the bus as it was transferring tourists from the airport.

He said six people died at the scene and another at the hospital. Two others were in critical condition.

Britain echoed the U.S. condemnation of the attack, with British Foreign Secretary William Hague offering condolences to the victims' families.

Late Wednesday, Israel announced it was dispatching a military medical and relief team to Bulgaria, a country of 7.3 million bordering Greece and Turkey.

The Burgas airport was closed and traffic redirected. In Sofia, meanwhile, Mayor Yordanka Fandakova ordered a stronger police presence at all public places linked to the Jewish community. There are some 5,000 Jews in Bulgaria and most live in the capital.