(CNN) -- A deadly explosion on a bus carrying Israeli tourists outside an airport in Bulgaria was "clearly a terrorist attack," Israeli Defense Minister Ehud Barak said Wednesday.
Key Israeli politicians pointed to Iran as the likely instigator, but Israel's U.S. ambassador acknowledged they had no proof.
Vania Valkova, director of the Bulgarian Interior Ministry press office, said that at least seven people were killed and about 30 wounded in the explosion in a parking lot outside Burgas Airport, on the Black Sea. The Foreign Ministry said six died and 32 people were wounded, with three in intensive care.
The attack was probably initiated by a group under the auspices of "either Iran or other radical Muslim groups," according to Barak, who cited Hezbollah and Hamas as likely suspects.
Black smoke fills sky after bus bombing
'We presume this is a terrorist attack'
"We are in a continued fight against them. We are determined to identify who sent them, who executed (the attack) and to settle the account," Barak said.
Bulgaria's foreign minister, Nikolay Mladenov, told CNN he presumes the explosion was a terrorist attack. But he said it was not time to "starting assigning blame."
Bulgaria will ask the U.N. Security Council to condemn the attack, perhaps Thursday, said Mladenov, who traveled to the blast scene.
The bus was scheduled to carry about 47 passengers to a resort. Bulgaria is a popular destination for Israelis.
Israeli Prime Minister Benjamin Netanyahu pointed to Iran, saying it has been behind a string of recent attempted attacks on Israelis in Thailand, India and Georgia, among others.
"All the signs (are) leading to Iran," he said, according to a statement from his office. "This is a global Iranian terror onslaught and Israel will react firmly to it."