Bulgarian officials said Tuesday they had evidence to suggest Hezbollah was involved in the July bombing in Burgas, Bulgaria, that seemingly targeted Israeli tourists.
Hezbollah holds Cabinet-level positions in the Lebanese government but maintains a military wing that's waged war on Israel in the past.
Kerry, whose government considers the entire Shiite movement a terrorist organization, called on European allies to respond by cracking 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," he said in a statement.
A spokesman for Catherine Ashton, Kerry's counterpart in the European Union, called for justice for those responsible for the July attack in Burgas.
"The implications of the investigation need to be assessed seriously as they relate to a terrorist attack on EU soil, which resulted in the killing and injury of innocent civilians" the statement read.
Hezbollah's terrorist designation isn't universal. Deputy Secretary-General Naim Qassem was quoted by The Daily Star newspaper in Lebanon as accusing Israel of "running a campaign of global terror against Hezbollah."