President Trump threatened on Monday to increase tariffs on all cars, trucks, car parts and steel from Canada to 50 percent, starting Jan. 1, as the relationship with America’s closest traditional ally edges toward a full-blown trade war.

Writing on Truth Social, the president said that Canada was “among the worst Nations in the World to deal with” and that it “will be treated like a State no longer.”

“They feel entitled, and yet, WE DON’T NEED CANADA, THEY NEED US!” he wrote.

Trade tensions between the United States and Canada have spiraled over the past few days, after the two sides failed to reach a trade deal and Mr. Trump imposed 50 percent tariffs on $20 billion of the country’s exports on Saturday.

Mr. Trump had threatened those tariffs on $20 billion of Canadian exports in a bid to speed trade talks and persuade Canada to remove some of the retaliatory measures it had taken against yet other tariffs Mr. Trump introduced last year.

On Tuesday, Mr. Trump said the countries were close to a deal and paused those tariffs for three days. But late on Friday, officials in Canada and the United States announced that they had failed to come to an agreement.

Mr. Trump’s actions provoked a defiant response from his Canadian counterparts. On Saturday, Mark Carney, the country’s prime minister, said the longtime allies were “at war” over trade, and promised to respond in kind with tariffs targeting U.S. agriculture, electronics, steel and other sectors.

Canadian officials attributed the breakdown to U.S. negotiators, claiming that Washington had made last-minute demands that upended the talks. But Jamieson Greer, the U.S. trade representative, said on Monday that Canadian leaders were to blame.

“We progressed to a point Tuesday night where we had enough agreement among the parties to announce that we had found a way to a deal,” Mr. Greer told CNBC. “Then we set about to finalize it. And then in the last hours, I think, there were things that the Canadians just, you know, they wanted more.”

Mr. Greer told The New York Times in an interview on Saturday that the U.S. offer included cutting tariffs targeting steel, aluminum and automobiles, and eliminating entirely a tariff Mr. Trump put on lumber last year. Speaking on CNBC on Monday, he said that Canadian officials had also been pressing U.S. leaders to drop some tariffs on heavy-duty vehicles.

“I don’t know if it was political for them,” Mr. Greer said. “It certainly doesn’t make economic sense, but perhaps for political reasons, they have some by-elections coming up, I don’t know. But they came in and they wanted more. And we weren’t prepared to do that.”