US President Donald Trump on Saturday threatened to exclude Canada from a new NAFTA agreement after negotiations to rewrite the pact ended without an agreement on Friday.
"There is no political necessity to keep Canada in the new NAFTA deal. If we don't make a fair deal for the U.S. after decades of abuse, Canada will be out," he tweeted.
"Congress should not interfere with these negotiations or I will simply terminate NAFTA entirely & we will be far better off."
US-Canadian trade talks are set to resume on Wednesday, and Ottawa and Washington still have time to reach an agreement that would keep Canada in the 25-year-old trade pact with Mexico.
But Trump's tweet seemed designed to greatly ramp up the pressure on Canadian negotiators.
Earlier in the week, officials had appeared close to reaching a deal that would rewrite the North American Free Trade Agreement, but leaked inflammatory comments from Trump angered Canadian officials and threatened to upend the talks.
The US and Mexico had announced a breakthrough accord on Monday, potentially leaving Canada out in the cold, but Ottawa's foreign minister Chrystia Freeland insisted that a "win-win-win" agreement was within reach.
The White House notified Congress on Friday of its "intent to sign a trade agreement with Mexico -- and Canada, if it is willing -- 90 days from now."
The White House now has 30 days to present the full text of a new agreement to Congress, which will give Ottawa and Washington time to iron out differences.
Trump's caution to Congress to "not interfere with these negotiations" appeared to refer to one potential obstacle: the fact that lawmakers have empowered him to reach a so-called "fast track" agreement only with both NAFTA partners, not just Mexico.
It is unclear how that will be sorted out.