The United States slapped a new anti-dumping tax on Canadian softwood lumber, the latest escalation in a trade dispute that began with an initial set of duties on exports from America's northern neighbor.

The US Commerce Department announced preliminary findings from an anti-dumping probe that Canadian lumber was 7.72 to 4.59 percent below normal prices.

US customs officers will now levy on Canadian timber exports these rates along with countervailing duties, bringing tax rates from 17.41 percent to 30.88 percent, depending on the business.

The US spared the provinces of Newfoundland and Labrador, Nova Scotia and Prince Edward Island from the additional taxes.

While US Secretary of Commerce Wilbur Ross said he was "optimistic that we will be able to reach a negotiated solution on softwood lumber," the measures were part of an effort to "stand up for American companies and their workers."

Canadian leaders said they were "deeply disappointed" by the latest US move in the trade spat, saying Canada "stands firmly behind the Canadian forestry industry."

"We will vigorously defend Canada's softwood lumber industry, including through litigation, and we expect to prevail as we have in the past," Canada's Foreign Minister Chrystia Freeland and Natural Resources Minister Jim Carr said in a joint statement.

Last year, imports of Canadian softwood lumber to the US were valued at an estimated $5.66 billion, according to the US Commerce Department.

Susan Yurkovich, who chairs the British Columbia Lumber Trade Council, denounced the tariffs as "without merit," saying they would also hurt US consumers looking to build or reconstruct.

And Jerry Dias, president of Canada's largest union Unifor, called the duties a "slap in the face to the concept of fair trade."