Sofia officials have yet to say publicly who they think organized the attack and Iran dismissed as unfounded Israel's accusations that it had played a role. The charges were "a familiar method of the Zionist regime, with a political aim, and is a sign of the weakness ... of the accusers," the Iranian embassy in Bulgaria said in a statement. Hezbollah has not commented on the bombing.
Israel's allegation, based on suspicions that Iranian and Hezbollah agents have been trying for years to score a lethal strike on its interests abroad, as well as investigations with the foreign countries involved, triggered speculation in local media that the Netanyahu government might now hit back hard.
The Israelis have long threatened to resort to military force to curb Tehran's disputed nuclear programme, but Defence Minister Ehud Barak sounded more restrained on Thursday about a response to the Bulgaria attack.
Speaking on Israel Radio, he said the country would "do everything possible in order to find those responsible, and those who dispatched them, and punish them" - language that appeared to suggest covert action against individuals.
President Shimon Peres said on his Facebook page that Israel would "take action in every terror nest, worldwide. It has the means to do so, and we are determined to act in this spirit."
In related developments, video surveillance footage of the bomber was released on Thursday. The footaged showed that the bomber was similar in appearance to tourists arriving at the airport, Interior Minister Tsvetan Tsvetanov said.
The bomber had been circling around a group of buses, which were about to take Israeli tourists to a resort near Burgas, for about an hour before the explosion, the surveillance footage showed.
"We have established there was a person who was a suicide bomber in this attack. This person had a fake driving licence from the United States, from the state of Michigan," Tsvetanov told reporters at the airport.
"He looked like anyone else - a normal person with Bermuda shorts and a backpack," he said. The bomber was said to be 36 years old and had been in the country for between four and seven days before the attack.
Special forces had managed to obtain DNA samples from the fingers of the bomber and were now checking databases in an attempt to identify him, Tsvetanov said. Bulgarian security services had received no indications of an imminent attack.
According to Burgas Mayor Dimitar Nikolov, bus driver Mustafa Kösöv (36), a Bulgarian citizen of Turkish origin, was among the victims of a bomb attack at a Bulgarian airport. Kösöv's dead body will be delivered to relatives living in the village of Yurukovo.
The foreign ministry said seven people were killed in the attack, including the Bulgarian bus driver and the bomber. The Israeli foreign ministry confirmed that five Israelis were killed.
The tourists had arrived in Bulgaria on a charter flight from Israel and were on the bus in the airport car park when the blast tore through the vehicle. Body parts were strewn across the ground, mangled metal hung from the double-decker bus's ripped roof and black smoke billowed over the airport.
On Thursday, the airport in Burgas - a city of 200,000 people at the centre of a string of seaside resorts - remained closed and police prevented people from approaching.
Beyond the cordons, about 100 holidaymakers waited for their flights but had been told they would be there until midnight. Officials were setting up portable toilets and tents for stranded travellers and Bulgaria's parliament opened with a one minute silence in memory of the bombing victims.
"It felt like an earthquake and then I saw flying pieces of meat," said Georgi Stoev, an airport official. "It was horrible, just like in a horror movie."
At Varna airport, also on the Black Sea, police with dogs were checking for explosives and tourists were asked to carry their luggage to their planes, national radio reported.
Wednesday's blast occurred on the 18th anniversary of a bomb attack on Argentina's main Jewish organisation that killed 85 people. Argentina blamed Iran, which denied responsibility.
Medical officials said two badly injured Israeli tourists were taken to hospitals in Bulgaria's capital Sofia. One woman was in intensive care with head and chest injuries and a man was in a critical state with burns covering 55 percent of his body.
About 70 Israeli tourists, including those lightly injured by the blast, left Burgas on a Bulgarian government airplane to Israel, the interior ministry said. The European Commission and NATO condemned the attack, joining criticism from the United States, Britain, France and Germany, a n d the mayor of Burgas announced a day of mourning. Israeli officials had previously said that Bulgaria, a popular destination for Israeli tourists, was vulnerable to attack by Islamist militants, who could infiltrate via Turkey.
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 that the Iranians have blamed on Israel and its Western allies.