Crucial trade talks between the United States and Canada to stave off punishing new tariffs by the Trump administration on Canadian goods collapsed late Friday, with Canada saying it would retaliate “dollar for dollar.”

United States trade representative Jamieson Greer told reporters that Canada had walked away from the negotiating table after days of talks had brought them close to an agreement that would avert 50 percent tariffs on more Canadian goods. President Trump had this week repeatedly said that the two countries, locked into a trade war since last year, were very close to a “good deal.”

“Tonight, Canada declined to finalize the trade deal under the terms agreed earlier this week, despite the U.S. offer to Canada to receive the best treatment of any major exporter to our market,” Mr. Greer said on a virtual briefing with the press.

“New demands and walk backs of other commitments by Canada have upended the careful balance reached in the past days,” Mr. Greer added.

Prime Minister Mark Carney of Canada said in a statement that it had been the American side that introduced last-minute terms that were “unfair, uneconomic, and called into question the reliability of any deal.”

He added that the talks had marked important progress but had ultimately “not been enough to meet our objectives for Canadians.”

Speaking to reporters, a U.S. official said that Canada wanted concessions that the United States wasn’t prepared to give, particularly when it came to the automotive sector, steel, aluminum and lumber.

If Canada does retaliate against the tariffs, the official said, Mr. Trump would be provided with options “to level out the playing field once again.”

Shortly before Mr. Greer’s statement to the press, the U.S. Customs and Border Protection sent out guidance for importers saying that any of the Canadian products that the president had identified would face tariffs after 12:01 a.m.

This is a developing story and will be updated.