WASHINGTON (TNND) — President Donald Trump sent a letter to Canada on Thursday threatening to raise taxes on imported goods to 35% in the on-again, off-again trade war with the country.

An original tariff of 25% was imposed on Canada in March in an alleged effort to get the country to crack down on fentanyl smuggling. Trump has also expressed frustration with a trade deficit with Canada that largely reflects oil purchases by America.

“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,” Trump wrote in the letter to Prime Minister Mark Carney.

The new tariffs are set to take effect on Aug. 1.

Carney responded to the letter on social media, saying, "Throughout the current trade negotiations with the United States, the Canadian government has steadfastly defended our workers and businesses."

"We will continue to do so as we work towards the revised deadline of August 1," he continued.

During Carney's visit to the White House in May, Trump told the prime minister there was nothing he could say that would get him to remove the tariffs.

When asked why, he said, "That's just the way it is."

Carney pushed back and said Canada is America's biggest client.

“You know, 50% of a car that comes from Canada is American. That’s not like anywhere else in the world," Carney said.

When Carney was elected prime minister in April, he told supporters, "President Trump is trying to break us so that America can own us. That will never, that will never, ever happen.”

Trump has sent letters to 23 countries so far informing them that the rates on their goods will increase if a trade deal is not made ahead of an Aug. 1 deadline.

A 10% base tariff rate began on "Liberation Day" in April for most countries, along with higher additional rates of up to 50%. Trump later delayed the effective date for 10% tariffs until July 9.

Since then, the president has tried to wield massive tariffs on other countries as a leverage point to get them to acquiesce to his administration’s demands and get better deals for American industries.

Only two trade deals have been put in place since the delay — one with the U.K. that left the most contentious issues unresolved and a newly announced deal with Vietnam.

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Editor's note: The Associated Press contributed to this article.