Justin Trudeau has warned Canada and the US could suffer a "thickening" border.
The US has announced duties of up to 24 per cent on softwood timber entering from Canada, and trade tensions between the two countries are escalating.
The Canadian Prime Minister said there are millions of good US jobs that depend on the smooth flow of goods, services and people back and forth across his border.
He said you can't thicken the border without hurting people on both sides of it.
Mr Trudeau has cited free trade in the North American automotive sector as an example of how a car part can cross the border up to six times before it ends up in a finished car.
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