Bulgarian President Rosen Plevneliev said he met the head of Mossad, Israel's intelligence service, about two months ago. "If we'd received any kind of information about this, it would have been taken very seriously," he told reporters.
"The fact that such an attack was carried out here and now should not be interpreted as to say that Bulgaria is not a safe country for tourists."
Bulgarian Interior Minister Tsvetan Tsvetanov said he was not excluding the possibility that the explosion was a terrorist attack.
"I can assure you that we're doing all we can to strengthen security in all the areas where it might be necessary to do so," he said.
U.S. President Barack Obama condemned what he called a "barbaric terrorist attack."
"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," said Obama, who called Netanyahu to express his condolences.
U.S. Secretary of State Hillary Clinton said the United States was prepared to assist Bulgaria and Israel in bringing those responsible to justice.
Neither she nor Obama mentioned Iran in their statements.
The tourists' plane from Tel Aviv landed in Bulgaria at 5 p.m., the Israeli Foreign Ministry said. The flight carried 154 passengers, eight of them children.
Oren Katz, who was with his wife and three children on a bus near the one that blew up, described a bloody scene.
"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," he said.
"We took our children with us and ran as far as we could away from the explosion. My oldest daughter is handicapped, so I decided to run back in order to pick up her wheelchair.
"There was a big blaze of fire, and we were not allowed to come near. Suddenly, I noticed an unconscious woman laying next to me, very close to the burning bus. I picked her up together with another man, and we managed to drag her out of the fire that was about to catch her body in seconds.
"I cannot forget the sight of body parts scattered around the bus."
The travelers were on seven buses outside the terminal; they had arrived on the same flight from Israel, Katz said.
The Israeli military said it would send a medical team to Bulgaria, which Oren called a hospitable vacation destination.
Bulgarian President Rossen Plevneliev:
"The fact that such an attack was carried out here and now should not be interpreted as to say that Bulgaria is not a safe country for tourists," said *
The explosion came on the 18th anniversary of an attack on a Jewish community center in the Argentinian capital, Buenos Aires, in which 85 people were killed.
Argentina's top prosecutor last year blamed Iran for the 1994 attack, which also wounded about 300 people. Israel also holds Iran responsible for the attack; Tehran has denied any connection to the bombing, which it has condemned.