WASHINGTON: There are signs Lebanon's Hezbollah may have orchestrated a suicide bombing attack on Israeli tourists in Bulgaria this week, a Pentagon spokesman said Friday.

“The attack does bear some of the hallmarks of Hezbollah but we're not in a position to make any final determination on who was responsible,” press secretary George Little told reporters.

“The Bulgarians are investigating,” he said.

The bomb that ripped through a bus Wednesday at Burgas airport on the Black Sea killed five Israeli tourists and injured more than 30, in the deadliest attack on Israelis abroad since 2004.

Israeli officials have publicly accused Iran and Hezbollah of carrying out the attack, though observers say it is unlikely to take military action against either Iran or Lebanon in response.

President Barack Obama has condemned the bombing as a “barbaric terrorist attack” and the White House said he spoke to Bulgaria's prime minister, offering US assistance in the investigation.

“We are working to assess the facts and, with our partners, to discover who was responsible,” White House spokesman Jay Carney told reporters aboard Air Force One en route to Florida. But Carney said he could not confirm if Hezbollah was behind the attack.

The New York Times quotes unnamed US officials saying the suicide bomber was part of a Hezbollah cell and that the assault was part of a broader “shadow war” between Israel and Iran.

With international pressure mounting on Tehran over its disputed nuclear program, the attack was meant as retaliation for the assassinations of Iranian nuclear scientists, which Iran has alleged were carried out by Israeli agents. Israel has neither confirmed nor denied the accusations.