The European Union and German automakers reacted with dismay Thursday after the US said tariffs on car imports could be on the horizon, potentially opening a new front in a burgeoning transatlantic trade conflict.

American Commerce Secretary Wilbur Ross on Wednesday raised the threat of car tariffs when he said he had initiated an investigation into whether auto imports "are weakening our internal economy and may impair the national security."

The move came after President Donald Trump tweeted there was "big news coming soon for our great American autoworkers".

The European Union, which has been lobbying feverishly to remain exempt from US border taxes on steel and aluminium ahead of a June 1 deadline, expressed surprise at the latest White House announcement.

"It is very difficult to imagine (car imports) create any sort of threat to the national security so it is very difficult to understand," European Commission Vice President Jyrki Katainen said in Brussels.

German car behemoth Volkswagen condemned Washington's "one-sided protectionism", saying "only free and fair trade secures increased prosperity".

The sentiment was echoed by German luxury carmaker BMW, which said "barrier-free access to markets" was key to global growth and employment.

With German carmakers set to be among those hit hardest the possible car tariffs, the German Chambers of Commerce and Industry (DIHK) said the US move "should almost be seen as a provocation".

According to the DIHK, autos and parts accounted for nearly 29 billion euros ($34 billion) of Germany's 111.5 billion euros in exports to the United States last year.

But although German carmakers exported nearly half a million vehicles to the US in 2017, they actually built over 800,000 vehicles at American factories where they employ some 36,500 people -- and car parts producers around 80,000 more.

Shares in Volkswagen, BMW and Mercedes-Benz maker Daimler were among the worst performers in the DAX index of blue-chip German shares in afternoon trading Thursday.

Global outcry

Imposing car tariffs would open yet another front in the Republican president's confrontational rows over trade that have drawn global outcry from allies and partners.

"Evidence of significant economic damage due to the trade conflict is mounting," tweeted economist Marcel Fratzscher of the DIW think-tank in Berlin.

"The Trump administration now adding new threats with tariffs on European cars could make things a lot worse."

The latest announcement comes as negotiations with Canada and Mexico over revamping the continent-wide North American Free Trade Agreement (NAFTA) have stalled over auto demands.

Trump had earlier blamed the US neighbours to the north and south for being "difficult" in talks to renegotiate the pact.

The contrast with a Thursday visit by German Chancellor Angela Merkel to Chinese premier Li Keqiang could not have been starker.

"China and Germany are on the path of promoting multilateralism and bolstering free trade," Merkel said in Beijing.

Japan was also quick to voice concern about the prospect of US car tariffs, with trade minister Hiroshige Seko saying such levies would "plunge the world market into confusion" and be "extremely regrettable."

Passenger cars make up around 30 percent of Japan's total exports to the United States and Tokyo has already threatened Washington with retaliation at the World Trade Organization for the steel tariffs.

'Great American autoworkers'

The Wall Street Journal reported earlier Wednesday that Trump was asking for vehicle import tariffs as high as 25 percent.

That would move US policy in the opposite direction from China, where President Xi Jinping recently offered to cut border taxes to 15 percent from 25 percent.

In its statement announcing the inquiry, the Commerce Department cited figures showing that US employment in automobile manufacturing had dropped by 22 percent from 1990 to 2017.

Trump -- whose protectionist platform helped launch him to the White House -- has repeatedly floated the notion of steep tariffs that would shield the US auto industry.

"After many decades of losing your jobs to other countries, you have waited long enough!" Trump tweeted.

He has specifically targeted Germany, and argued that American cars are slapped with higher tariffs than those imposed on European autos.

US cars sold in the EU are hit with 10 percent duties, while the US imposes just 2.5 percent on cars from the EU.

But Washington imposes 25 percent tariffs on European pick-ups and trucks -- which the EU taxes at a much lower 14 percent on average.