The Trump administration is set to impose a fresh 50% tariff on some Canadian goods under a never-before-used legal provision, citing what it said was unfair treatment by Ottawa of American alcohol, automobile and dairy products.

The new tariffs will take effect in 30 days, according to a senior administration official who briefed reporters Monday on the decision. Certain key imports such as energy, potash, fish, critical minerals and goods covered by separate sectoral duties on industries, including autos and metals, will be excluded.

Crucially, the official said there would be no exemptions for products under the existing North American trade pact between the U.S., Canada and Mexico. President Donald Trump signed a proclamation ordering the tariffs earlier Monday, the official said.

Trump has threatened tariffs on other nations’ goods before, only to pull back on some of them after negotiations or negative market consequences.

The tariffs are being applied under Section 338 of the 1930 Tariff Act, which gives the president the power to impose duties of as much as 50% from countries deemed to discriminate against U.S. commerce. The provision has never before been used to impose tariffs.

Monday’s action threatens to further strain relations between the two neighbors, traditionally close allies.

Trump last week threatened to increase tariffs on Canadian products to punish the country for wildfire smoke that covered U.S. cities, including New York and Washington, over the last week. The senior administration official said the U.S. was still exploring options linked to Trump’s threat, that but Monday’s action was unrelated to wildfires.

Further complicating the trade relationship between the two countries, the U.S. earlier this month declined to extend its trade agreement, known as USMCA, with Canada and Mexico, setting up what could be years of contentious negotiations.