Speaking on Thursday, Nathan Sales, the US State Department's coordinator for counterterrorism, accused the Iran-backed group of smuggling and storing chemicals, including ammonium nitrate, from Belgium to France, Greece, Italy, Spain and Switzerland.
"It stores these weapons in places so it can conduct major terrorist attacks whenever its masters in Tehran deem necessary," Sales said in a briefing without elaborating or providing evidence of the activities.
Ammonium nitrate is an industrial chemical commonly used in fertilizers and as an explosive used for quarrying and mining. It is considered relatively safe if uncontaminated and stored properly.
It is extremely dangerous if contaminated, mixed with fuel or stored unsafely, as was seen at the Beirut port in August after 2,750-tonnes of it exploded, destroying the port and killing at least 190 people.
"To our knowledge, there is nothing tangible to confirm such an allegation in France today," foreign ministry spokeswoman Agnes von der Muhll told reporters in response to Sales' allegations.
Unlike the United States, which has designated the heavily armed and politically powerful Hezbollah movement as a terrorist group since 1997, France has said its elected arm has a legitimate political role.
France is spearheading efforts to set Lebanon on a new course after decades of corrupt rule that has led to its deepest crisis since the 1975-1990 civil war.
Its European allies, Germany and Britain, also consider the group a terrorist organization, but French officials argue that ostracizing it would make efforts to resolve the crisis impossible.
Sales said the chemicals had been smuggled into Europe in first-aid kits and were possibly in Spain, Greece and Italy.
"Any illegal activity committed by a foreign organization on our territory would be sanctioned by the French authorities with the greatest firmness," von der Muhll said.