WASHINGTON -- U.S. President Donald Trump has delivered his most sweeping broadside at Canada, blasting the northern neighbour for trade practices he says must be corrected in three areas: energy, lumber, and dairy.

Trump suggested he would have more details to share within a couple of weeks about his government's plans for the North American Free Trade Agreement -- but signalled clearly in the meantime that he intends to play hardball.

"We can't let Canada or anybody else take advantage and do what they did to our workers and to our farmers," Trump said in the Oval Office.

"Included in there is lumber, timber and energy. We're going to have to get to the negotiating table with Canada very, very quickly."

It's unclear what specifics he was referring to. On energy, the current NAFTA guarantees the U.S. a fixed rate of Canada's oil production without any import fees. On lumber, cheaper Canadian wood has reduced the cost of U.S. homes but also caused recurring legal spats with the U.S. industry that alleges product-dumping.

He was a bit clearer on dairy. Trump made it obvious his complaints from earlier this week in Wisconsin were specifically about recent rule changes on milk classification, not on the longer-term issue of Canada's supply-management system.

"Canada, what they've done to our dairy farm workers, is a disgrace. It's a disgrace," he said. "Rules, regulations, different things have changed -- and our farmers in Wisconsin and New York state are being put out of business."

It could all be up for discussion in NAFTA and other forums.

As it prepares to enter continental trade negotiations this fall, the U.S. is weighing other actions that affect Canada.

The U.S. is debating the idea of a border tax that could hurt certain imported products -- including Canadian oil, which is a leading source of America's energy supply. Decisions are also expected in the coming weeks on duties on Canadian lumber, another instalment of a once-a-decade feud between Canadian and American lumber producers.

Trump made the remarks while in the process of signing an executive order on U.S. steel.

Indeed, they came at the end of a longer statement, when he said, "I wasn't going to do this," before launching into a pointed complaint that echoed sentiments he heard this week about dairy in Wisconsin.

Ironically, one of Trump's guests in his office for the signing ceremony was the head of the United Steelworkers union -- Leo Gerard, a Canadian, from Sudbury, Ont.