The United States will impose preliminary anti-subsidy duties averaging 20% on imports of Canadian softwood lumber, Commerce Secretary Wilbur Ross said on Monday, escalating a long-running trade dispute between the two neighbors.

The move, which affects some $5.66 billion worth of imports of the construction material, sets a tense tone as the two countries and Mexico prepare to renegotiate the 23-year-old North American Free Trade Agreement.

Canada denounced the U.S. action and vowed to protect its lumber interests through litigation.

News of the tariffs sent the U.S. dollar sharply up against the Canadian dollar in Asian trading to hit an almost four-month high. The Canadian currency sank to C$1.3559 to the greenback, or 73.75 U.S. cents, down from its North American close of C$1.3516, or 73.99 U.S. cents.

Ross told Reuters in a telephone interview that Canada was “already retaliating” against the United States well ahead of the lumber duties by restricting imports of U.S. highly filtered milk protein products used by cheesemakers.

President Donald Trump last week called Canadas dairy protections “unfair.”

Ross said some Wisconsin dairy producers were now “losing their farms” because of the restrictions. “Apparently Canadians now are coming down and saying: Since you can’t do it anymore, I’ll buy your equipment for 5 cents on the dollar,'” he said.

U.S. lumber producers asked the Commerce Department last November under President Barack Obama to investigate what they viewed as unfair subsidies to Canadian competitors who procure their timber from government lands at cheaper rates. U.S. lumber producers generally cut timber grown on private land.

Canadian Natural Resources Minister Jim Carr and Foreign Minister Chrystia Freeland said in a joint statement that Commerce’s accusations “are baseless and unfounded” and would raise U.S. home construction and renovation costs.

Ross said the duties collected would total about $1 billion a year. In a statement, he said the need for the lumber duties and Canadas dairy restriction were “not our idea of a properly functioning free trade agreement.”

NAFTA never addressed the softwood lumber issue or Canadas largely closed dairy market. The Trump administration has vowed to renegotiate NAFTA on terms that would reduce U.S. goods trade deficits of $63 billion with Mexico and $11 billion with Canada last year.

NAFTA TALKS EXPECTED THIS SUMMER

Ross said NAFTA’s dispute resolution system needed to be changed because it had worked against the United States in the lumber dispute.

NAFTA talks are expected to begin later this summer after a 90-day legal consultation period.