"This action highlights Hezbollah's activities within Syria and its integral role in the continued violence the Assad regime is inflicting on the Syrian population," the US Treasury Department said in a statement.
Washington already classes Hezbollah a "terrorist organization" and it is under US sanctions, but Friday's move explicitly ties the group to the violence underway in Syria, where Assad is attempting to put down a revolt.
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"Hezbollah's extensive support to the Syrian government's violent suppression of the Syrian people exposes the true nature of this terrorist organization and its destabilizing presence in the region," said David Cohen, the Treasury's under secretary for terrorism and financial intelligence.
"Long after the Assad regime is gone, the people of Syria and the entire global community will remember that Hezbollah, and its patron Iran, contributed to the regime's murder of countless innocent Syrians."
Hezbollah was added to a blacklist associated with an executive order signed by US President Barack Obama in August last year which targeted the government of Syria and its supporters.
Those sanctions were designed increase pressure on Damascus as Washington called for the first time for Assad to step down over his military assault on rebelling Syrians opposed to his rule.
But 17 months after the start of the uprising the Syrian leader remains in power, and more than 20,000 people have been killed.
