Washington (AFP) - US President Donald Trump took a crack Monday at the head of the nation's largest federation of labor unions on the US Labor Day holiday after the workers' advocate criticized his threat to exclude Canada from a new NAFTA deal.

Speaking Sunday on Fox News, AFL-CIO president Richard Trumka said it was "pretty hard to see" how Trump's efforts to revamp the North American Free Trade Agreement would work "without having Canada in the deal."

"We've been aggressively pursuing an agreement that works for the workers in all three countries and I can say we're not done yet," Trumka told the Fox network, a favorite among Trump's base.

The US president -- who said last week there's "no political necessity" for Canada to remain in the trade pact -- responded to Trumka with a jab on the final day of America's three-day holiday weekend celebrating the worker.

"Some of the things he said were so against the working men and women of our country, and the success of the U.S. itself, that it is easy to see why unions are doing so poorly," the US president tweeted of Trumka.

Minutes later Trump took on a more jovial tone, tweeting "Happy Labor Day!"

"The U.S. has tremendous upside potential as we go about fixing some of the worst Trade Deals ever made by any country in the world."

In the Fox interview, Trumka also had charged that Trump has "used his office to actively hurt working people."

The AFL-CIO collectively represents 12.5 million workers, and as its head Trumka is an influential voice on trade issues.

In the ongoing NAFTA negotiations, the US and Mexico announced a tentative agreement last week, potentially leaving Canada out in the cold -- although Ottawa's foreign minister Chrystia Freeland insisted 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."

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.