European Commission chief Jean-Claude Juncker on Friday said the EU would hit back with tariffs of its own if US President Donald Trump made good on threats to slap duties on foreign cars, as an EU-US trade truce wobbled.

"We agreed with Trump on a kind of ceasefire when it comes to new car tariffs," Juncker told German broadcaster ZDF. But if the US were to break the agreement, "then we will do the same".