Donald Trump's 25% tariffs on goods from Mexico and Canada have come into effect, as has an additional 10% on Chinese products, bringing the total import tax to 20%.
The US president confirmed the tariffs in a speech at the White House - and his announcement sent US and European stocks down sharply.
The tariffs will be felt heavily by US companies which have factories in Canada and Mexico, such as carmakers.
Mr Trump said: "They're going to have a tariff. So what they have to do is build their car plants, frankly, and other things in the United States, in which case they have no tariffs."
There's "no room left" for a deal that would see the tariffs shelved if fentanyl flowing into the US is curbed by its neighbours, he added.
Mexico and Canada face tariffs of 25%, with 10% for Canadian energy, the Trump administration confirmed.
And tariffs on Chinese imports have doubled, raising them from 10% to 20%.
Canada announced it would retaliate immediately, imposing 25% tariffs on US imports worth C$30bn (£16.3bn). It added the tariffs would be extended in 21 days to cover more US goods entering the country if the US did not lift its sanctions against Canada.
China also vowed to retaliate and reiterated its stance that the Trump administration was trying to "shift the blame" and
"bully" Beijing over fentanyl flows.
Mr Trump's speech stoked fears of a trade war in North America, prompting a financial market sell-off.
Stock market indexes the Dow Jones Industrial Average and the Nasdaq Composite fell by 1.48% and 2.64% respectively on Monday.
The share prices for automobile companies including General Motors, which has significant truck production in Mexico, Automaker and Ford also fell.
? Follow Trump 100 on your podcast app ?
The losses continued on Monday. Asian markets closed down, and European markets opened lower.
The pan-European Stoxx 600 dropped 1.07% while the biggest indexes in major European economies fell sharply.
Consumers in the US could see price hikes within days, an expert said.
Gustavo Flores-Macias, a public policy professor at Cornell University, New York, said "the automobile sector, in particular, is likely to see considerable negative consequences".
This is due to supply chains that "crisscross the three countries in the manufacturing process" and " because of the expected increase in the price of vehicles, which can dampen demand," he added.
Read more:
The effects of Trump trade tariffs explained
Trump hits out at Zelenskyy again
The Trump administration is gearing up to bring in other tariffs in the coming weeks.
On 2 April, reciprocal tariffs will take effect on all countries that impose duties on US products.
He is also considering 25% tariffs on goods from the EU "very soon" after claiming the bloc was created to "screw the United States".
(c) Sky News 2025: Donald Trump confirms Mexico and Canada tariffs - prompting a stock market sell-off
Trump treads fine line between striking a deal and blackmailing Ukraine
Kim Jong Un's sister threatens response to US aircraft carrier's deployment in South Korea
How Trump's tariffs could cost consumers in the US and UK - even if he spares Britain
More than 30 killed in Bolivia bus crash - second deadly collision within days
The Pope has had 'two episodes of acute respiratory failure', Vatican says
Long-range drone strikes weakening Russia's combat ability, senior Ukrainian commander says
