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.

"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.

Another spokeswoman for the Interior Ministry said an explosive device may have been on the bus, but authorities cannot be sure. A forensic team has begun an investigation, she said.

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."

Michael Oren, Israel's ambassador to the United States, said his country was investigating the circumstances.

"We don't have proof of Iranian involvement but it is part of this pattern" of plots around the world, he told CNN's "The Situation Room."

The Iranian government had no immediate comment on the incident.

Bulgarian authorities also vowed to find out what happened.

Israel's intelligence minister told CNN's Christiane Amanpour that Israel has "very good indications" Iran was involved.

"They are after us, Israelis, wherever they can find them," said Dan Meridor.

Venelin Petkov, a reporter for bTV in Bulgaria, said that three of the wounded were in critical condition. The Bulgarian bus driver was among the dead, according to Valkova of the Bulgarian Interior Ministry.

The Israelis had just arrived from Tel Aviv and were to have traveled to a beach resort about 50 kilometers (30 miles) away, he said.

Security expert Ivan Boyadjiev told Bulgarian National Television that Israel had warned several months ago of the possibility of an attack on Israelis in Bulgaria.

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."