A suicide bombing that killed Israeli tourists in Bulgaria this week bore hallmarks of Iranian-backed Hezbollah militants but the U.S. Defense Department has not yet concluded who was behind it, a Pentagon spokesman said on Friday.

The attack on a bus carrying Israelis at a Bulgarian airport, "does bear the hallmarks of Hezbollah," George Little, the Pentagon press secretary, told reporters.

Bulgaria's interior minister, Tsvetan Tsvetanov, said earlier in the day that the attacker, who killed himself and six others, was a foreign national. Sofia was investigating with the help of foreign intelligence services, he said.

Little turned aside a request to characterize the signs of an Hezbollah attack or how it could be distinguished from one by, for instance, al-Qaida, which is not linked to Iran.

Prime Minister Benjamin Netanyahu has accused Hezbollah militants of carrying out the bombing Wednesday at Burgas airport, a popular gateway for tourists visiting Bulgaria's Black Sea coast. Iran has denied any involvement.