Six people were confirmed dead and 32 wounded Wednesday in a terrorist bombing targeting Israeli tourists in a Bulgarian Black Sea port of Burgas, DPA reported.
Foreign Minister Nikolay Mladenov announced the toll, saying that three people hospitalized in Burgas were in intensive care. He promised that Bulgaria would "do everything possible" to clarify the background of the attack.
Israeli leaders accused Iran of organizing the attack and pledged a firm response.
The blast hit a bus due to take some 40 Israeli tourists, arriving on a flight from Tel Aviv, from the Serafovo airport in Burgas to a Black Sea resort 30 kilometres away.
Bulgarian and Israeli authorities immediately said the Burgas blast was a terrorist attack.
In Tel Aviv, Prime Minister Benjamin Netanyahu blamed Iran for the airport attack and promised that Israel would retaliate "forcefully to the Iranian terrorism."
"All the signs lead to Iran. Over the past months we have seen Iranian attempts to hurt Israelis in Thailand, India and Georgia, Kenya, Cyprus and more," Netanyahu charged.
Defence Minister Ehud Barak promised that Israel would go after the planners.
"We have been following for a while plans by terrorist organizations - Hezbollah, Hamas, Iranian elements and the Jihad - to carry out terrorist attacks around the world," he said. "The security establishment will act with all its force to reach the perpetrators of this attacks and those who sent them."
Barak telephoned with US Defence Secretary Leon Panetta and discussed the possibility of a joint investigation by the US Central Intelligence Agency and Israel's Mossad.
The bomb ripped through a bus parked at the Serafovo airport at 5:30 pm (1430 GMT). The coach was due to take some 40 Israeli tourists, who had arrived 40 minutes earlier on an Air Vin flight from Tel Aviv, to the Sunny Beach resort on the Black Sea coast.
Witnesses said that the explosion scattered body parts around the car park and set three buses on fire.
"The moment we got on (the bus), we heard a very loud explosion. It was the third bus next to us. Everyone started running in all directions. There was a big chaos," tourist Oran Katz told CNN.
"There was a big blaze, and we were not allowed to come near," he said. "I cannot forget the sight of body parts scattered around the bus."
Reports quoted witnesses who said they saw someone board the coach seconds before the blast, fueling speculation that the attack may have been a suicide bombing, but there was no official confirmation.
Bulgaria closed the Burgas airport in the wake of the bombing but was allowed Israeli aircraft to land to assist and fly the wounded home. Two planes sent by the nation's medical service have left Israel, and a military flight was due to take off shortly.
Also flying to Bulgaria were volunteers of Zaka, the Israeli non-governmental religious organization which deals with collecting body parts after terrorist attacks. Under Jewish religious law, the entire body has to be buried.
In Washington, US President Barack Obama offered condolences to Netanyahu in a telephone call following the attack and vowed to provide whatever assistance was needed to bring the perpetrators to justice, stressing the long-standing US friendship with Israel.
"The president strongly condemned this outrageous attack that killed and wounded innocent Israelis and Bulgarians, including Israeli children," Obama spokesman Jay Carney said.
The president of the Toronto-based Centre for Israel and Jewish Affairs, David Koschitzky, pointed to the significance of the date.
"It is important to note that today marks the anniversary of the notorious 1994 attack on a Jewish community centre in Buenos Aires - the worst bombing in Argentina's history," he said.
The Buenos Aires attack 18 years ago killed 85 people. The Argentine foreign ministry late Wednesday expressed its "profound solidarity with Israeli people and government, in particular with survivors and victims' relatives."
The bombing also came weeks ahead of the 40th anniversary of the hostage-taking at the Munich Olympic Games in which 11 Israeli athletes were killed by Palestinian terrorists.
Wednesday's attack and the two anniversaries, Koschitzky said, were a clear reminder that "the one constant of terrorism is that Jews around the world remain a favoured target."