U.S. President Donald Trump on Friday fired back at Canada after Ottawa and other American allies retaliated against Washington's steel and aluminum tariffs, and appeared to threaten possible action against Canada's lumber industry.

"Canada has treated our Agricultural business and Farmers very poorly for a very long period of time. Highly restrictive on Trade! They must open their markets and take down their trade barriers! They report a really high surplus on trade with us. Do Timber & Lumber in U.S.?" Trump tweeted.

Canada has treated our Agricultural business and Farmers very poorly for a very long period of time. Highly restrictive on Trade! They must open their markets and take down their trade barriers! They report a really high surplus on trade with us. Do Timber & Lumber in U.S.?

— Donald J. Trump (@realDonaldTrump) June 1, 2018