President Donald Trump threatened Thursday to impose a 35% tariff on Canadian imports, upending negotiations between the countries that had Canada’s representatives hopeful that a trade deal could be reached in a matter of weeks.

Trump posted a letter to Prime Minister Mark Carney of Canada on social media outlining his latest threat. He said the new, higher tariff rate for Canada would go into effect Aug. 1, though it was unclear if it would affect all Canadian goods, or if he would follow through.

Trump’s letter to Canada resembled the format of letters he has sent to nearly two dozen American trading partners this week. But it differed from the others by accusing Canada of having “financially retaliated” against the United States with its own tariffs. It also raised Trump’s repeated assertion — unsupported by American and Canadian data — that Canada had not done enough to stop the flow of fentanyl across the U.S.-Canada border, as well as his complaints about Canada’s high tariffs on U.S. dairy farmers.

Trump also suggested that the new tariffs were subject to further negotiations before the Aug. 1 deadline. “These Tariffs may be modified, upward or downward, depending on our relationship with your Country,” Trump concluded.

Tariffs of 35%, if applied widely, could harm Canada’s export-dependent economy, which relies heavily on the United States, its biggest trading partner.

In a late-night post on social media, Carney said Canada would try to get the best deal for its workers in the talks, acknowledging the new deadline but not the fresh threat of higher tariffs.

“Canada has made vital progress to stop the scourge of fentanyl in North America,” he said.Earlier this year, Trump imposed a 25% tariff on imports from Canada but then exempted most products that qualify as North American under the U.S.-Mexico-Canada Agreement, the trade pact he signed in his first term.