US President Donald Trump expanded his offensive against trading partners on Tuesday, threatening 25 percent tariffs on imported cars, and similar or higher duties on pharmaceuticals and semiconductors.

Trump has announced a broad range of levies on some of the biggest US trading partners since taking office in January, arguing that they will help tackle unfair practices – and in some cases using the threats to influence policy.

He recently pledged 10 percent duties on all goods coming from China, and 25 on steel and aluminium imports.

At his Mar-a-Lago resort in Florida, he told reporters that tariffs on the automobile industry will "be in the neighbourhood of 25 percent," with specifics to come around April 2.

Asked about threatened tariffs on pharmaceuticals and chips, Trump said: "It'll be 25 percent and higher, and it'll go very substantially higher over (the) course of a year."

He added he wanted to give affected companies time to bring their operations to the United States, saying that he had been contacted by major firms that "want to come back."

The president also said that Washington's trading partners could avoid being taxed by investing in factories in the United States.

"We want to give them time to come in," he said. "When they come into the United States and they have their plant or factory here, there is no tariff. So we want to give them a little bit of a chance."

Experts have warned it is often Americans who end up paying the cost of tariffs on imports, rather than foreign exporters.

About 50 percent of the cars sold in the United States are manufactured within the country. Among imports, about half come from Mexico and Canada, with Japan, South Korea and Germany, also major suppliers. (AFP)