Chief Executive Jean-Pierre Clamadieu said Solvay had set up factories in countries such as China and the United States assuming that they could easily ship to other markets.

"The fact that all of a sudden barriers are popping up leads us to ask questions, such as, can we keep investing in the United States?," Clamadieu told Reuters.

"If we see that a trade war is setting in for the long haul, it will put into question our strategies for locating" operations, he said, speaking on the sidelines of an economics conference in Aix-en-Provence in southern France attended by many French CEOs.

U.S. President Donald Trump's administration, which has already imposed tariff hikes on steel and aluminium made in the EU, Canada and Mexico, has threatened a tariff increase on all EU-assembled vehicles.

French Finance Minister Bruno Le Maire insisted on Sunday that Europe would retaliate to further hikes, although Germany has signalled that it is prepared to accept lower EU import car tariffs to ease tensions with Washington, something Paris is unlikely to accept.

Clamadieu, a Frenchman who was named chairman of French energy group Engie in May, said it was clear that there were risks of cracks appearing in Europe's united response to the United States.

"Europe has institutional tools, but what it is missing is strong leaders capable of hitting a fist on the table with as much strength as needed," he said.

(Reporting by Pascale Denis; writing by Leigh Thomas; editing by Jason Neely)