UPDATE 6-Israel blames Iran for Bulgaria bus bomb that kills 6

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

* Blast on 18th anniversary of Argentina attack

* Obama says attack "completely outrageous"

By Stoyan Nenov

BURGAS, Bulgaria, July 18 (Reuters) - Six people were killed in a bomb attack on a bus carrying Israeli tourists at a Bulgarian airport on Wednesday and Israel accused Tehran of carrying out the attack, promising a strong response to "Iranian terror."

Body parts were strewn across the ground and mangled metal hung from the bus's torn-back roof. Its windows were blown out and surrounded by scorch marks, as clouds of dense black smoke billowed above the airport.

"We heard a very strong blast. The bus was full with people and children. Flesh and blood everywhere," a Bulgarian witness told TV7 television. "I saw another bus catching fire from the one that exploded. It was complete chaos."

The tourists had arrived on a charter flight from Israel and were on the bus in the car park outside Burgas airport when the blast ripped through the double-decker. By late evening, the airport was still sealed off and closed as authorities tried to pin down exactly what had happened.

"We sat down and within a few seconds we heard a huge boom and we ran away. We managed to escape through a hole on the bus," Aviva Malka told Israeli Army Radio from the scene.

Bulgarian Foreign Minister Nikolai Mladenov told Reuters by telephone after visiting the airport: "The explosion was caused by a bomb in the bus."

The Interior Ministry declined to comment on whether it might have been a suicide attack, as some witnesses had speculated, and said it was questioning people who had been close to the blast.

The blast 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.

Israeli Prime Minister Benjamin Netanyahu blamed Iran 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.

VULNERABLE TARGET

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 Turkey.

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

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

Although Tehran has denied involvement, some analysts believe it is trying to avenge the assassinations of several scientists from its 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 rein in its programme. Israel and Western powers accuse Iran of working towards a nuclear bomb, but Tehran says its research is strictly for peaceful ends.

Israel Foreign Minister Avigdor Lieberman said the bomb could have been stuck on the side of the bus - the way an attack on an Israeli embassy car in India was carried out in February.

"The Revolutionary Guard are behind the attack ... I don't want to say unequivocally, but they are behind it directly or indirectly," Lieberman told Channel 2 television, without elaborating on the Iranian military unit.

Bulgaria raised security at all airports, bus and railway stations after the explosion. Sofia Mayor Yordanka Fandakova also boosted policing at public places linked to the Jewish community in Bulgaria's capital.

The blast damaged several other buses in the car park outside the airport at Burgas, where they were waiting to ferry tourists to resorts along the Black Sea coast. Stunned travellers hugged one another in shock at the carnage and passengers were kept away from the scene with a police cordon.

"COMPLETELY OUTRAGEOUS"

Bulgaria's Foreign Ministry said six people had been killed, including one who died in hospital. Thirty-two people were in hospital, three of whom were in intensive care.

Israel said seven people had been killed. Bulgarian Police have declined to give the nationality of the dead. A TV station in Bulgaria said a Bulgarian may have been killed.

Israeli Rabbi Haim Tverdovich based in Burgas told Israeli Channel 2 some of the wounded had suffered severe burns.

U.S. President Barack Obama called the attack "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," he said.

Obama communicated that message directly to Netanyahu in a telephone call, the White House said, and B ulgarian Prime Minister Boiko Borisov also vowed in a call to his Israeli counterpart to find the perpetrators and bring them to justice.

Israel has sent security officials to help with the investigation and its military was sending two planes to Burgas, one with medical crews to provide assistance to the injured and fly them home and another with forensic experts. A third plane with medics from the emergency services was also on its way.

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.

"Such a terrifying act on the territory of a sovereign country, member of the EU, is a provocation to the efforts of the democratic community for achieving world peace," said Bulgarian Prime Minister Boiko Borisov.

The chief mufti of Bulgaria's Muslims, who account for about 10 percent of the Balkan country's 7.4 million population, also condemned the attack, as did the EU and British and French foreign ministers.

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

Israel's flag carrier El Al cancelled its flight from Tel Aviv to Sofia that was due to leave at 1600 GMT a spokeswoman told Reuters. Nothing had been decided about Thursday's flights.

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