Canadian goods imported into the U.S. will face a blanket 35 percent tariff starting next month, President Donald Trump said Thursday in an announcement that came in the midst of active trade negotiations between the two countries.

Canada, one of the largest U.S. trading partners, is the latest in a series of countries that have received a letter from Trump in recent days warning of new tariffs set to take effect Aug. 1. Futures for the major stock indexes fell on the news.

The rate announced by the president in a social media post is higher than expected and marks an increase from the 25 percent he set on Canadian goods in February.

A White House official, granted anonymity to discuss the negotiations, said the administration plans to impose the tariff only on goods that do not comply with the 2020 USMCA, though the ultimate details will be up to Trump to decide.

Trump suggested in an earlier interview with NBC that he was considering tariffs of only 15 or 20 percent on all countries that had not reached a trade deal with the U.S.

In a letter addressed to Prime Minister Mark Carney and shared on his social media platform, Truth Social, Trump criticized Canada for what he called its “failure” to prevent drugs from being smuggled into the U.S.

The president has repeatedly said the tariffs are intended as retaliation for Canada not doing enough to prevent fentanyl from crossing the border — though statistics show that relatively little of the drug enters the U.S. by that route.

“If Canada works with me to stop the flow of fentanyl, we will, perhaps, consider an adjustment to this letter,” Trump wrote.

Canadian Foreign Minister Anita Anand said Thursday, before Trump’s announcement, that she was hopeful the two countries would reach a new economic and security agreement by July 21, a deadline that Carney had been pushing.

“Our teams are working extremely hard behind closed doors in a complex negotiation,” Anand said. “It is necessary for the health of our economy and the benefit of Canadian workers in Canadian businesses.”

In the NBC interview, Trump said he would also notify the European Union of new tariffs on its 27 member countries.

The EU and the U.S. have been locked in high-stakes negotiations for months to reach a trade deal and avoid tariffs on hundreds of billions worth of each other’s goods.

Maroš Šefčovič, the EU’s top trade negotiator, told European lawmakers on Wednesday that negotiations were ongoing and had so far “spared the EU” from one of Trump’s letters.

Ari Hawkins contributed to this report.