Bulgarian President Rossen Plevenliev speaks during a press conference after a national security conference in Sofia on February 5, 2013. The government said two people linked to Hezbollah were behind a bomb attack on Bulgaria's Black Sea coast in July that killed five Israeli tourists. (Dimitar Dilkoff/AFP/Getty Images)

The Obama administration sought Tuesday to increase pressure on Europe to brand Hezbollah a terrorist group after the Bulgarian government implicated the militants in a fatal attack on Israeli tourists last summer.

Bulgaria blamed the Iran-backed group for the July bus bombing in the Black Sea city of Burgas, making official what U.S. and Israeli terrorism officials had alleged from the start. The attack killed five Israelis and their Bulgarian driver.

Bulgaria said evidence showed that two of the people involved in the attack were members of Hezbollah’s military wing and that they were acting as part of a campaign against Israeli targets worldwide. The United States and Israel assisted with the investigation.

The allegation of a direct Hezbollah terror campaign on European soil escalates pressure on the European Union to reconsider its treatment of Hezbollah. The E.U. has resisted past U.S. and Israeli entreaties to designate Hezbollah a terrorist group.

Secretary of State John F. Kerry issued a statement urging Europe to crack down on Hezbollah. “We need to send an unequivocal message to this terrorist group that it can no longer engage in despicable actions with impunity,” said Kerry.

Several influential members of the 27-nation E.U. alliance have argued, however, that Hezbollah is both a political and military organization and that a blanket terrorism designation could be counterproductive. Hezbollah supporters move through Europe and raise money with little obstacle.

Kerry discussed the Bulgarian allegation in a telephone conversation with E.U. foreign policy representative Catherine Ashton, said Victoria Nuland, the State Department spokeswoman. Nuland said Ashton is “well aware” of the U.S. position on Hezbollah but would not say whether Kerry specifically lobbied for a change.

But, Nuland said, “our hope and expectation is that this clear evidence of Hezbollah operation on European soil will be galvanizing to their internal conversations.”

Tsvetan Tsvetanov, the Bulgarian interior minister, said at a news conference in Sofia, the capital, that two of the three attackers had genuine passports from Australia and Canada. He said there was evidence that they belonged to Hezbollah’s military wing and had been financed by the group.

No one has been arrested in connection with the attack.

Ashton’s spokesman, Maja Kocijancic, said the E.U. would discuss the implications of Hezbollah’s involvement in an attack on European soil.

The United States labeled Hezbollah a terrorist organization in 1997, which bars the group from using U.S. banks and prohibits most diplomatic contact.

“Our concern is that in the context of our squeezing them, they look for other places to do their banking, to do their plotting, et cetera,” Nuland said. “And our concern has been that Europe has been one of the places that they have exploited.”

White House counterterrorism chief John Brennan urged Europe to “take proactive action to uncover Hezbollah’s infrastructure and disrupt the group’s financing schemes and operational networks in order to prevent future attacks.”

Brennan did not expressly call on the E.U. to revisit the question of terrorism designation, but others did.

“This brazen act of terrorism by Hezbollah was committed on European Union soil,” said Sen. Robert P. Casey Jr. (D-Pa.), who with 74 other senators last year wrote to the E.U. urging action. “Brussels can no longer delay.”