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"For many, many months, we've made no secret of our desire to see the EU take firmer action with regard to Hezbollah," said State Department spokeswoman Victoria Nuland, adding the issue had been repeatedly raised in talks.
The Lebanon-based Muslim Shiite group Hezbollah has been on a US terror blacklist since 1995 after a series of anti-American attacks, including the bombing of the US embassy and Marine barracks in Beirut in the 1980s.
However, the militia -- which is an ally of Syrian leader President Bashar al-Assad and funded by Iran -- is also the most powerful faction in Lebanon's cabinet, controlling 18 out of 30 seats, and is the country's most powerful military force.
Washington has been actively pushing the European Union to take steps to ban Hezbollah as a terrorist organization as well.
The US fears that, as it has been "squeezing" Hezbollah, the group has looked for "other places to do their banking, to do their plotting," Nuland said.
"Our concern has been that Europe has been one of the places that they have exploited."
The bus bombing, in which five Israeli tourists and their Bulgarian driver were killed at Burgas airport, was part of "a stepped-up terrorist campaign by Hezbollah... around the world over the past year," she warned.
New Secretary of State John Kerry discussed the issue during talks Tuesday with EU foreign policy chief Catherine Ashton.
And President Barack Obama's top counter-terrorism advisor John Brennan, his nominee to lead the CIA, called on European states to take "proactive action" to uncover Hezbollah's infrastructure, financing and operational networks.
Brennan said the attack exposed Hezbollah as "a terrorist group that is willing to recklessly attack innocent men, women, and children, and that poses a real and growing threat not only to Europe, but to the rest of the world."
But US officials praised Bulgaria for its professional and comprehensive investigation into the attack, vowing Washington would stand with the government in Sofia as it fights terrorism.
Israeli Prime Minister Benjamin Netanyahu also said the European Union should draw the "necessary conclusions" about Hezbollah following the investigation, hinting the group should be placed on a terror watch list.
Nuland acknowledged however there were differences among EU states about how to deal with Hezbollah, given that it is part of an elected government.
"There are varying views about whether... there's a difference between the military wing and the political wing. You know that our view is that we don't recognize a distinction, but different governments have different views," she said.