Japan's government on Thursday warned of a "significant impact" on its economic ties with the United States -- and on global trade -- after US President Donald Trump announced import tariffs on cars.

Tokyo is a close economic and strategic US ally, with its firms the biggest investors in the United States and one in 10 Japanese jobs are connected to the auto sector.

Trump announced on Wednesday 25 percent tariffs "on all cars that are not made in the United States" effective 12.01am eastern time on April 3.

Japanese Prime Minister Shigeru Ishiba, who last month held friendly talks with Trump at the White House, said Japan was reviewing an "appropriate" response.

"Japan has made significant investments and significant job creation, which doesn't apply to all countries... we are the number one (country) in investment in the United States," Ishiba added.

"The US president's understanding on this is significantly deepening. However, various things are happening, so we are considering all kinds of countermeasures," he said.

Chief government spokesman Yoshimasa Hayashi went further, calling the tariffs "extremely regrettable" and warning of major effects.

"We believe that the current measures and other broad-based trade restrictions by the US government could have a significant impact on the economic relationship between Japan and the US, as well as on the global economy and the multilateral trading system," Hayashi told reporters.

"In response to this announcement, we have again told the US government that this measure was extremely regrettable and we strongly urged the US government to exclude Japan from the scope of this measure," Hayashi said.

Trump's announcement hammered carmakers in Asia.

The world's top-selling automaker Toyota at midday was down 2.7 percent, while Nissan shed 2.6 percent and Honda was off 2.7 percent.

In South Korea, Hyundai shares dipped 4.0 percent. (AFP)