There is more work to be done, Trudeau said, calling the recent announcement that General Motors will close plants in Canada and the U.S. "a heavy blow".
The United States, Canada and Mexico signed a North American trade pact on Friday, and President Donald Trump predicted an easy passage through U.S. Congress to approve the deal, despite opposition from some lawmakers.
The new agreement requires that 75 percent of each vehicle granted duty-free treatment be made in North America - up from 62.5 percent under NAFTA - and requires a significant amount of manufacturing be performed by workers earning at least $16 an hour. And Trump is facing a House of Representatives controlled by the Democrats.
"There's much more work to do in lowering trade barriers and in fostering growth that benefits everyone", Trudeau said. US intelligence officials have concluded that the Saudi crown prince must have at least known of the plot to kill Khashoggi, who was critical of the Saudi royal family.
The United States-Mexico-Canada Agreement, the proposed successor to NAFTA, would allow the first $150 (Canadian) of goods bought in the USA and shipped to Canada to be duty-free.
Trump signed the agreement with Canadian Prime Minister Justin Trudeau and Mexican President Enrique Pena Nieto, on his last day in office as Mexico's leader. But he said he could support it with a "few tweaks that move it in the direction of a more pro-trade agreement".
- President Trump opened two days of diplomacy at the Group of 20 summit in Argentina on Friday after his abrupt decision to cancel a meeting with Russian President Vladimir Putin overshadowed the proceedings before they even started.
Mr Pena Nieto, who will be replaced on Saturday with Mexico's newly-elected president, has sold the deal to his people as a victory, saving free trade from a U.S. president who wanted to see it killed.
Simply put, "The NAFTA 2.0 text is not the transformational replacement of the corporate-rigged trade-pact model that progressive activists, unions, and congressional Democrats have long demanded", wrote Lori Wallach, director of Public Citizen's Global Trade Watch.
TRUMP: "These new provisions will benefit labour, technology and development in each of our nations, leading to much greater growth and opportunity throughout our countries and across North America".
It didn't take long before it was noticed that the acronym added an A to 'USMC, ' the United States Marine Corps. The U.S. continues to push for a quota in exchange for lifting the tariffs on Canada and Mexico, which have applied their own retaliatory levies. Unmet in the new deal, they say, are their demands to "restore local and national sovereignty over farm and food policy; stop corporate giveaways in trade agreements; and ensue economic viability and resilience in rural communities".
Trump also expressed optimism about a deal on trade with China, telling reporters "There's some good signs".
Both Trudeau and Trump have taken criticism over the new agreement.
"Make no mistake, we will stand up for our workers and fight for their families and their communities", he said, before addressing Trump directly.
Trump was greeted warmly at the Casa Rosada by Argentine President Mauricio Macri, a longtime business acquaintance.