Washington DC - US President Donald Trump declared on Saturday that he "couldn't care less" if automakers increase car prices for Americans in the wake of his new import tariffs.

There have been reports that Trump threatened auto executives with reprisals if prices jump, but he told NBC News that increasing prices would simply help US-based manufacturers.

"I couldn't care less. I hope they raise their prices because if they do, people are gonna buy American-made cars. We have plenty," he told NBC host Kristen Welker.

On Wednesday, Trump imposed a blanket 25% import tariff on cars and light trucks made outside the US, due to take effect on April 3.

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Tariffs will be delayed for car parts from countries covered by the US trade pact with Mexico and Canada as officials try to disentangle the mixed supply chain.

But otherwise, Trump intends for the import levy to be permanent, in order to boost US production and, in his view, save the American auto industry.

Despite his boosterism, however, share prices of the biggest US automakers have suffered and experts have warned that price rises will hit American consumers.

Asked by NBC News what his message would be to worried auto executives, Trump said, "The message is 'congratulations,'"

"If you make your car in the United States, you're going to make a lot of money."