HARRISBURG, Pa. -- U.S. President Donald Trump has again raised the spectre of the U.S. pulling out of the North American Free Trade Agreement, saying America has been on the "wrong side" of the trade pact for "many, many years."

Trump told a rally in Harrisburg, Pa., on Saturday night that he'll try to renegotiate the agreement with Canada and Mexico, but will terminate NAFTA if a "fair deal" for the U.S. can't be reached.

Trump also noted that his administration has ordered the collection of "billions and billions of dollars" in duties on imports from countries he says "break the rules."

The U.S. has already imposed tariffs of up to 24 per cent on Canadian softwood timber and is investigating whether steel and aluminum imports pose a threat to national security.

Trump told the crowd he was prepared to announce a few days ago that the U.S. was leaving NAFTA until he got calls from Prime Minister Justin Trudeau and Mexican President Enrique Pena Nieto asking to negotiate.

The Financial Times reported Friday it received a leaked draft executive order that Trump had originally planned to sign on Saturday that would have given five days notice of the U.S. intention to leave NAFTA.

A copy of the draft order posted online by the Financial Times says Canada has "continued to exploit the American dairy and lumber industries."

The document also blames NAFTA for a "massive transfer of wealth" from the United States, the loss of 700,000 jobs and a more than $1-trillion trade deficit with Mexico since the agreement was enacted in 1993.

"We have been on the wrong side of the NAFTA deal with Canada and with Mexico for many, many years, many decades -- we can't allow it to happen," Trump told Saturday's rally. "If we can't make a fair deal for our companies and our workers, we will terminate NAFTA."

On Thursday, Trump hinted that economic disruption was another reason for not announcing a U.S. pullout from NAFTA.

The mere rumour of it happening, floated on Wednesday by the White House, shaved almost two per cent off the Mexican peso and a third of a cent off the loonie.

The leaked draft executive order sets a high bar for trade agreements, saying they must "increase America's economic growth, decrease America's trade deficit, raise American wages, maintain the integrity of America's borders, and strengthen the manufacturing and defence industrial bases of the United States."

Trump drew cheers from the people at the rally when he reminded them of his "buy American and hire American" policy.