WASHINGTON • The United States has accused its close ally and neighbour Canada of "rough" trade practices that have hurt American workers and farmers.

The verbal attack on Canada - which was reinforced by US announcements of sweeping tariffs on Canadian lumber imports - has few parallels in modern history and drew an immediate rebuke from the other side of the border, triggering fears of a trade war between the two major economies.

In a sharp pivot from his criticism of China and Mexico for their trade practices, US President Donald Trump suggested that Canada's benign image was deceptive, and the country's policies were damaging and unfair to the United States.

"People don't realise Canada's been very rough on the United States. Everyone thinks of Canada being wonderful and civil," Mr Trump told a roundtable meeting of farmers at the White House on Tuesday. "I love Canada. But they've outsmarted our politicians for many years, and you people understand that."

Canadian Prime Minister Justin Trudeau said the country would protect its industries. "I'm polite, but I'm also very firm in defending Canada's interests," he told the broadcaster CTV.

Mr Trump's comments followed a 45-minute televised assault on Canadian trade practices by US Commerce Secretary Wilbur Ross, as well as an announcement of new tariffs targeting Canada's important lumber industry.

Mr Ross, who appeared at the White House media briefing, took a similar hard line against Canadian trade policies regarding dairy products and lumber - the two commodities at the heart of the trade dispute.

Mr Ross said the twin disputes underline the problems with the North American Free Trade Agreement (Nafta) linking the US with Canada and Mexico, which Mr Trump has vowed to renegotiate.

"If Nafta were functioning properly, you wouldn't be having these kinds of very prickly and unfortunate events back to back," Mr Ross said. "They are a close ally. They're an important ally. They're generally a good neighbour. That doesn't mean they don't have to play by the rules."

Later on Tuesday, Mr Trump and Mr Trudeau discussed the trade issues in a "very amicable" phone call, the White House said.

The escalation - in words and deeds - represented the biggest rupture in relations since 2003, when Canada said it would not take part in the US-led invasion of Iraq.

And it raised concerns about unintended consequences on both sides of the border. Canadian officials warned that US home builders would be hit hard by the new lumber tariffs, as they could cause a spike in the cost of supplies they use for construction.

But the Trump administration's newly aggressive rhetoric earned praise from Republican and Democratic lawmakers in lumber and dairy states.

Democratic Senators Amy Klobuchar and Al Franken of Minnesota celebrated the lumber tariffs, calling them "welcome relief" for workers and rural communities in their state. Governor Scott Walker and House Speaker Paul Ryan, both Wisconsin Republicans, praised the move to protect dairy farmers.

Canadian lawmakers have also begun to dig in. During a briefing on Tuesday, some Canadian officials raised the possibility of a legal challenge to the softwood lumber tariff under Nafta or the World Trade Organisation, although neither could happen until next year, after the Commerce Department makes its final determinations.

THE WASHINGTON POST, AGENCE FRANCE-PRESSE

SEE WORLD

WATCH THE VIDEO

Trudeau vows to defend Canada's interests http://str.sg/4ByC