Asked during his appearance with Trudeau whether NAFTA was dead, Trump said, "We'll see what happens".
Trudeau told reporters in Washington DC that he agreed with the U.S. president that that the NAFTA trade deal needed to be revamped and that any new agreement needed to "give citizens opportunities to succeed".
The meeting, part of a bilateral "CEO dialogue" that meets a couple of times each year, included a closed-door discussion on the NAFTA negotiations addressed by Foreign Minister Luis Videgaray and Economy Minister Idelfonso Guajardo, who are in charge of the negotiations for Mexico.
Harper says he understands the frustration: he described his annoyance at spending his 50th birthday signing a bailout package for General Motors Canada, only to see the auto giant move jobs out of the country. Replying to a question, Trump said he would consider a trade pact with Canada minus Mexico, adding that both the USA and Canada wanted to protect their workers.
The US president said if there was no deal on the NAFTA, it would be terminated.
The U.S. Chamber of Commerce on Tuesday accused Trump's administration of trying to sabotage the talks with "poison pill proposals", including demands for more favorable treatment for the U.S. side on auto production, and a "sunset clause" to force regular negotiations.
Trump and Trudeau met as negotiators from the three NAFTA countries gathered in the Washington suburb of Arlington to get the next round of trade talks under way.
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One of the most contentious US proposals is around so-called rules of origin for vehicles, which govern what share of a auto must be built within NAFTA countries to receive the pact's benefits. Representatives of the auto industry warned some of the USA demands could raise prices for vehicles.
The proposals call for North American content, overall, to rise to 85 percent from the current 62.5 percent. The negotiations were extended on Wednesday by two days to Oct 17.
The hard issue of rules of origin will be addressed mostly at the end of the current talks, according to a schedule obtained by Reuters.
Mexico and Canada do some $20 billion a year in bilateral trade.
"If the required content to hit the threshold for a NAFTA vehicle is too high, people may say, 'Look, it's just too hard, it's too high, so we'll just ship the vehicles in, '" Magna International Inc.
Jerry Dias, president of Unifor, Canada's largest labor union, said it was clear the United States does not want a deal.
Despite their common ground, however, Canada and Mexico are also at odds on some key issues.
"My optimism toward NAFTA, toward a renegotiation, isn't based on personality or reading political tea leaves", Trudeau said following his talks with Trump.