A bomb detonated in a bus leaving the Bulgarian airport on Wednesday killed seven people, which included five Israelis and left at least 30 people seriously wounded.
The bombing was "clearly a terrorist attack," according to Israeli Defense Minister Ehud Barak, and he, along with other Israeli politicians, has cited Iran as the most probable suspect, CNN said.
Israeli Prime Minister Benjamin Netanyahu directly accused Iran of the attack and said that "Israel will respond forcefully to Iranian terror," BBC News reported.
U.S. President Barack Obama and U.S. Secretary of State Hillary Clinton both released statements expressing the United States' remorse and support for Israel, but neither made any mention of Iran, CNN said.
The Iranian Government has not released any immediate comment on the bombing, CNN said.
Witnesses described the bus exploding shortly after it was boarded by passengers. One woman said the explosion had taken place in the front of the bus, BBC News said.
Bulgarian journalist Dobromir Dovkacharov arrived to the scene 30 minutes after the attack and described three buses as "completely burnt-out."
Dovkacharov relayed what some eyewitnesses had seen, which included flames, decapitated heads and human limbs, BBC News said.