US President Donald Trump signed an order on Saturday to impose steep tariffs on imports from Mexico, Canada and China, fulfilling a campaign promise but raising the prospect of higher prices for American consumers.

Trump is declaring an economic emergency to impose tariffs of 10% on all imports from China and 25% on imports from Mexico and Canada - America's largest trading partners - with the exception of a 10% tariff on Canadian oil.

The White House said Trump's order also includes a mechanism to escalate tariffs if countries retaliate against the US, as they have threatened.

Trump says the tariffs will force countries to do more to stop the flow of fentanyl into the U.S., but they also align with his embrace of protectionist measures to boost domestic production and as a potential source of revenue for the federal government.