US President Donald Trump said Thursday that he might increase tariffs on imported automobiles in the near future, in a bid to shore up domestic manufacturing.Trump has imposed 25 percent levies on foreign automobiles since returning to the presidency this year, and he said: I might go up with that tariff in the not too distant future. The higher you go, the more likely it is they build a plant here, he added at a bill signing ceremony.US auto shares slumped shortly after his remarks, with shares in Ford down 1.6 percent and those for General Motors sliding 1.5 percent.Trump's auto tariffs have cast a pall over the industry. GM last month projected a hit of up to $5 billion to its 2025 earnings due to the president's levies.The US auto giant has described talks with the Trump administration as productive, as the White House tempered tariffs on auto parts.But its expected impact includes costs from duties on finished vehicles from South Korea, where GM manufactures a number of value-priced autos.Apart from tariffs on imported vehicles, Trump has also slapped fresh levies on steel and aluminum, alongside sweeping duties on various trading partners -- adding to pressures on businesses.