US President Donald Trump has imposed a 50% tariff on a wide range of goods imported from Canada, in retaliation for what he called "unequal treatment" of US cars, dairy and alcohol.

The president said punishing 50 percent tariffs would begin by Aug. 19, a significant escalation of the trade war between Canada and the U.S. that he started over a year ago.

Trump signed a series of executive orders Monday that would impose a 50 per cent tariff on a range of goods, from honey to hockey sticks, with no exemptions under the North American free trade pact.