UPDATE 3-Bus blast in Bulgaria kills 4, Israel blames Iran

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* More than 30 injured in explosion in Black Sea city

* Israel PM says "all the signs lead to Iran"

* Blast on 18th anniversary of Argentina attack

By Tsvetelia Tsolova

SOFIA, July 18 (Reuters) - At least four Israeli tourists were killed on Wednesday in an explosion on a bus outside a Bulgarian airport that Israel blamed on Tehran, promising a strong response to "Iranian terror".

The explosion comes on the 18th anniversary of a 1994 bomb attack on the headquarters of Argentina's main Jewish organisation by an Iranian-backed Hezbollah suicide bomber, which killed 85 people.

The windows of the double decker bus were blown out and surrounded by scorch marks. Mangled metal hung from its torn-back roof and clouds of dense black smoke smoke billowed above the airport.

The mayor of Burgas, on Bulgaria's Black Sea coast, said the bus was carrying Israeli tourists, but police could not immediately confirm their nationality. Several other buses at the site were damaged.

"I do not know what it was, but it was a very powerful blast, and I think it was something placed on purpose in the bus, which carried 47 Israeli tourists," Burgas mayor Dimitar Nikolov told BTV television, adding 33 people were injured.

An Israeli witness told Israeli army radio that the explosion was probably caused by a suicide bomber at the entrance of the bus.

Bulgaria raised security at all airports, bus and railway stations after the explosion, which happened in a parking lot outside the airport. Stunned travellers hugged one another in shock at the carnage and passengers were kept away from the scene with a police cordon.

Israeli Prime Minister Benjamin Netanyahu blamed Iran for the Wednesday's blast and said Israel would respond.

"All the signs lead to Iran. Only in the past few months we have seen Iranian attempts to attack Israelis in Thailand, India, Georgia, Kenya, Cyprus and other places," Netanyahu said in a statement.

"Eighteen years exactly after the blast at the Jewish community centre in Argentina, murderous Iranian terror continues to hit innocent people. This is an Iranian terror attack that is spreading throughout the entire world. Israel will react powerfully against Iranian terror," he said.

The incident was not reported by Tranian media and there was no immediate Iranian reaction to the israeli accusation.

VULNERABLE

Israeli officials had previously said that Bulgaria, a popular holiday destination for Israeli tourists, was vulnerable to attack by Islamist militants who could infiltrate via nearby Turkey.

Burgas is Bulgaria's fourth largest city and lies on the Black Sea coast some 60 km (40 miles) from the border with Turkey. It is at the centre of a string of seaside resorts which are popular for their sunshine and low cost compared with many parts of the Mediterranean.

With a population of about 200,000, it is also an important industrial centre and has Bulgaria's sole oil refinery.

"The latest information is that four people have died. Three on the scene and one later in hospital. Over 30 people were rushed to hospital," an Interior Ministry spokeswoman said.

Israeli media said seven people had been killed.

Israeli diplomats have been targeted in several countries in recent months by bombers who Israel said struck on behalf of Iran.

Though Tehran has denied involvement, some analysts believe it is trying to avenge the assassinations of several scientists from its controversial nuclear programme, which the Iranians have blamed on Israel and its Western allies.

Israel has threatened air strikes against Iran's nuclear facilities if diplomatic efforts fail to stop Tehran getting nuclear weapons, which it denies it is seeking.

Washington was still trying to make sure of the facts, White House spokesman Jay Carney said.

"I don't have information yet on anything specific to the incident itself, if in fact it was terrorism and who was responsible for it," Carney said.

Burgas airport was closed after the incident and flights were redirected to the airport of Varna, police said. Dozens of tourists were stranded at the airport as it was checked for other explosive devices, Focus news agency reported.

El Al cancelled its flight from Tel Aviv to Sofia that was due to leave at 1600 GMT and consequently the turnaround flight, a spokeswoman told Reuters. Nothing has been decided about Thursday's flights.

Israeli rescue services were planning to send a plane to Burgas with medical staff to treat the injured and take bodies home, the Israeli ambulance service Magen David Adom said.

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