President Donald Trump threatened to double tariffs on Canadian cars on Monday, the latest sign that the United States and Canada are barreling toward a deeper trade war after last-ditch efforts to strike a deal collapsed Friday.

The failed efforts resulted in President Donald Trump enacting 50% tariffs on roughly $20 billion worth of Canadian goods early Saturday morning. Canada is preparing to retaliate with “dollar-for-dollar” tariffs starting September 8, Prime Minister Mark Carney announced over the weekend.

And on Monday, Trump escalated matters further, threatening to double duties charged on Canadian cars and car parts to 50% come January 1.

“Canada has been ripping off the United States of America for years,” Trump said in a Truth Social post. “Not sustainable, and NOT ANYMORE!”

In his post Trump also threatened to increase steel duties to 50% on January 1; however, that’s the current tariff rate for those products.