Marathon trade talks between the United States and Canada to stave off fresh tariffs by the Trump administration on Canadian goods collapsed late Friday, a top United States official said, leading to the introduction of 50 percent levies on more Canadian goods.

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. 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.

The Canadian government did not immediately have a comment.

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

This is a developing story and will be updated.