Washington DC - President Donald Trump on Thursday threatened to unleash tariffs of 35% on imports from Canada starting on August 1 as he continued his attack on the US' neighbor to the north.

"Starting on August 1, 2025, we will charge Canada a Tariff of 35% on Canadian products sent to the United States, separate from all Sectorial Tariffs," Trump said in a letter addressed to Canadian Prime Minister Mark Carney shared on Truth Social.

"If for any reason you decide to raise your Tariffs, then, whatever the number you choose to raise them by, will be added onto the 35% that we charge," he continued.

In the one-page letter, Trump again accused Canada of failing to stop the influx of fentanyl into the US, even though data shows only a marginal amount of the deadly drug comes into the country over the northern border.

Donald Trump US State Department starts carrying out layoffs of more than 1,300 employees

Donald Trump Brazil summons US envoy over Trump's "interference" in Bolsonaro coup case

"I must mention that the flow of Fentanyl is hardly the only challenge we have with Canada, which has many Tariff, and Non-Tariff, Policies and Trade Barriers, which cause unsustainable Trade Deficits against the United States," the Republican added.

The US is by far Canada's most important trading partner, but a historically close relationship has been tested to the limits since Trump took office for a second time, threatening annexation and mocking Canada's leader's as "governors" of a 51st state. The two countries have been engaged in tense trade negotiations since Carney took over from his predecessor, Justin Trudeau.

Over the past week, the 79-year-old has been sending letters to a host of countries, including key allies Japan and South Korea, telling them to expect higher tariffs unless a new trade deal can be struck by the end of the month.