WASHINGTON - US Commerce Secretary Wilbur Ross said on Monday his agency will impose new anti-subsidy duties averaging 20 percent on Canadian softwood lumber imports, a move that escalates a long-running trade dispute between the two countries.

Speaking to Reuters by phone a day ahead of the expected announcement, Ross said that the duties would affect $5 billion worth of softwood lumber imports from Canada.

"It's about 31.5 percent of the total US market, so it's a pretty big deal in terms of the Canadian relationship," he said.

The trade case was filed at Commerce against Canadian lumber producers by their US competitors, who argued that Canadian timber grown on public lands receives an effective subsidy from the government that is unavailable to the US firms, who cut timber grown largely on private land.

A Commerce Department fact sheet on the pending announcement shows that West Fraser Mills will pay the highest duties at 24.12 percent, followed by Canfor Corp at 20.26 percent.

Resolute FP Canada Ltd will pay a 12.82 percent duty, while Tolko Marketing and Sales and Tolko Industries will pay 19.50 percent duty and J.D. Irving Ltd will pay 3.02 percent.

All other Canadian producers face a 19.88 percent duty, according to the document.

The determination directs US Customs and Border Protection to require cash deposits for the duties on all new imports as well as softwood products imported over the past 90 days.

However, the duties need to be finalized by Commerce and then confirmed by the US International Trade Commission.