Kevin O'Leary: "This is the END of Canada..."

The US is Canada's top trade partner, buying more than $400bn in goods last year under a longstanding free trade agreement.

But Trump hit that trade with a new 25% tariff earlier this year, citing concerns about drug trafficking at the border.

New US tariffs on cars, steel and aluminium have also scrambled relations. Car parts, for example, cross US, Mexican and Canadian borders multiple times before a vehicle is completely assembled and such import taxes threaten supply chains.

Trump later carved out exemptions for some goods in the face of widespread alarm from businesses in both the US and Canada, which has hit back with tariffs of its own on some US products.

Shares in the US fell on Friday after Trump said he was cutting off talks, but later bounced back with the S&P 500 closing at a record high.