A suicide bomber who blew up a bus in Bulgaria last week, killing five Israeli tourists, was backed by an organized group who helped him plan and carry out the attack, Bulgarian Prime Minister Boiko Borisov said on Tuesday.

Borisov said police had not yet identified the bomber, who along with the Bulgarian bus driver also died. More than 30 people were wounded in Wednesday's attack at Burgas airport.

Borisov said the perpetrator had not acted alone.

"These are extremely experienced people who have followed strict conspiracy rules," Borisov told reporters after meeting John Brennan, a U.S. counter-terrorism adviser to President Barack Obama.

"From what we see, they arrived nearly a month beforehand, changed rental cars, and traveled to different cities ... and not more than one of the people we are looking for was captured on either security camera," Borisov said.

He declined to give more details on the plotters.

"There was absolutely no chance of preventing such an act of violence," Borisov insisted, according to The Associated Press. "We could have only detected it by chance or if we had been informed by the services that such activities were under way in Bulgaria."

Borisov said that the bomber's DNA and fingerprints had not matched anything held on file by Bulgaria or by partner spy agencies.

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He suggested that the attacker, whose bomb was concealed in his backpack, may have entered Bulgaria on a plane from the European Union's "Schengen" passport-free travel zone.

"We do not know his identity, but it is known when he has arrived, the presumed flight, where he came from. It could turn out that he entered Bulgaria from a Schengen member country," Borisov said.

Israel's military chief insisted Tuesday that Iran and Hezbollah were involved and vowed that Israel would respond to the attack.

"We will have to find a way to respond to this attack, and not just a one-off," Lt. Gen. Benny Gantz was quoted as telling the Israeli parliament's foreign affairs and defense committee. "We will know how to do it judiciously. Ultimately, the response will come."

His comments were relayed by a meeting participant who discussed contents from the closed session on condition of anonymity. Iran has denied the accusations.

Borisov said that Bulgaria -- a member of both the European Union and NATO -- would not say who it thought was responsible for the attack until the investigation was complete.

Brennan said the U.S. has been working with Bulgaria on the investigation.

"Bulgaria will continue to have the full support of the United States in the weeks and months ahead," Brennan said.

Reuters and The Associated Press contributed to this report.

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Updated at 6:04 p.m. ET: SOFIA, Bulgaria -- An explosion on a bus carrying Israeli tourists at an airport in Burgas killed at least six people and injured 32 others, Bulgarian authorities said. Bulgarian officials could not confirm the deadly blast was terror-related but Israeli Prime Minister Benjamin Netanyahu blamed Iran.

"Iran is responsible for the terror attack in Bulgaria, we will have a strong response against Iranian terror," said Netanyahu in a statement, according to Haaretz, an Israeli newspaper.

Tehran did not immediately issue a comment.

A bomb caused the explosion, Bulgarian Foreign Minister Nikolai Mladenov told Reuters.

The Israelis had landed at the airport around 4:45 p.m. local time (9:45 a.m. ET) and boarded a bus to their hotel when the explosion happened, The Jerusalem Post reported. Body parts were flung onto the tarmac and a thick black plume of smoke rose above the airport.

An 11-year-old Israeli girl and two pregnant women were among those injured, according to Focus, a Bulgarian news agency.

EPA

Smoke rises over the Burgas airport in Bulgaria, after an explosion on Wednesday.

"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. Burgas is 250 miles from Sofia, Bulgaria's capital.

Nikolov said 171 people had arrived on a plane from Israel to spend their holiday at the Black Sea coast. One American and one Slovenian passenger were on board, he said.

The Bulgarian Press Office, which provided the casualty figures, said only one bus was involved in the explosion, but added the investigation is ongoing. 

According to a Bulgarian news service, an eyewitness named Daniel told the Voice of Israel radio program: “I was literally watching people crawling out of the bus. They were screaming and one of them had no arms or legs. It was horrible.”

Another Israeli traveler told the radio station: “The people who survived got through the windows and were trying to crawl over the bodies. The bus was destroyed from both sides.”

In separate statements, President Barack Obama, Secretary of State Hillary Clinton and United Nations Secretary-General Ban Ki-moon condemned the attack.

Reuters

An explosion damaged a bus carrying Israeli tourists at Burgas Airport in Bulgaria on Wednesday.

"As Israel has tragically once more been a target of terrorism, the United States reaffirms our unshakeable commitment to Israel's security, and our deep friendship and solidarity with the Israeli people," Obama said. He called Netanyahu on Wednesday to express his condolences.

Clinton said the U.S. is prepared to offer "any assistance necessary" and that she was prepared to "work with our partners in Bulgaria, Israel and elsewhere so that the perpetrators can be apprehended swiftly and brought to justice for this appalling crime."

Wednesday's bombing coincided with the 18th anniversary of the bombing of a Jewish community center in Argentina that killed 85 people. According to the BBC, Argentinian prosecutors charged Iran with orchestrating the attack, which they believe was carried out by Hezbollah, the Lebanon-based militia. 

Netanyahu 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. "This is an Iranian terror attack that is spreading throughout the entire world. Israel will react powerfully against Iranian terror."

But Jerusalem Post writer Yaakov Katz questioned the connection between the two events.  

"While the attack is severe, it is not of the scale of what happened in 1994," Katz wrote. In 1994 in Argentina, a van filled with explosives rammed into the Jewish community center, killing 84 people. Wednesday's attack, he said, appeared to have been perpetrated by a suicide bomber or a planted bomb. 

"This is a break from Hezbollah's traditional tactic of carrying out attacks with less of a footprint," Katz wrote. "In previous plots that were thwarted recently, there were attempts to shoot down Israeli airliners with shoulder-to-air missiles, to plant bombs on diplomatic cars or to assassinate Israeli diplomats. Nothing that would leave evidence behind."

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

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 program, 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.

The Israel Airports Authority announced disruptions in flights from Israel and Europe, according to Haaretz.

NBC's Lawahez Jabari in Jerusalem, Reuters and The Associated Press contributed to this report.

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