Canada will face a 35 percent tariff on exports to the United States starting August 1, President Donald Trump said Thursday in a letter to Prime Minister Mark Carney.

It was the latest of more than 20 such letters issued by Trump since Monday, as he continues to pursue his trade war threats against dozens of economies.

Canada and the US are locked in trade negotiations in hopes to reach a deal by July 21 and the latest threat seemed to put that deadline in jeopardy.

Canada and Mexico are both trying to find ways to satisfy Trump so that the free trade deal uniting the three countries - known as the USMCA - can be put back on track.

The United States-Mexico-Canada Agreement replaced the previous NAFTA accord in July 2020, after Trump successfully pushed for a renegotiation during his first term in office.

It was due to be reviewed by July next year, but Trump accelerated the process by launching his trade wars after he took office in January.

Canadian and Mexican products were initially hard hit by 25 percent US tariffs, with a lower rate for Canadian energy.

Trump targeted both neighbors saying they did not do enough on illegal immigration and the flow of illicit drugs across borders.

But he eventually announced exemptions for goods entering his country under the USMCA, covering large swaths of products. Potash, used as fertilizer, got a lower rate as well.

The letter on Thursday came despite what had been warming relations between Trump and Carney.

The Canadian leader came to the White House on May 6 and had a cordial meeting with Trump in the Oval Office.

They met again at the G7 summit last month in Canada, where leaders pushed Trump to back away from his punishing trade war.