A U.S. official says the Iranian-backed militant group Hezbollah is believed to have been behind the suicide bombing that killed at least seven people on a bus full of Israeli tourists in Bulgaria.

The official spoke to The Associated Press on the condition of anonymity to discuss sensitive intelligence. He did not elaborate.

Israeli Prime Minister Benjamin Netanyahu has said the bombing was carried out by Hezbollah, and Israel has vowed to strike back.

Iran has called the accusation "baseless."