Delegates from the United States, Mexico and Canada on Tuesday wrapped up a fifth round of talks on renegotiating the North American Free Trade Agreement -- but pushed off some of the thornier issues to later meetings.

Their efforts, this time held in Mexico City, are aimed at reforming their NAFTA pact enough to head off US President Donald Trump's threats to scrap it.

The latest round focused on trade remedies, rules of origin and matters related to agriculture, Mexico's chief technical negotiator, Kenneth Smith, told reporters.

More complicated subjects "are going to be left for the following meetings," he said.

Various US proposals were put on the table, as were some Mexican ones, while specific questions were raised in other areas, Smith explained.

Meanwhile, "some concepts that are unacceptable" were put forward regarding rules of origin in the automobile industry, he said.

The US wants to see domestic US components in cars rise to 50 percent, to the ire of Mexico and Canada.

Trump, who espouses an "America first" policy, views the US trade deficit with Mexico in particular as a "disaster" for American jobs and prosperity. 

"I’ve been opposed to NAFTA for a long time," Trump said last month, as he hosted Canadian Prime Minister Justin Trudeau at the White House. 

"If we can't make a deal, it'll be terminated and that will be fine," he said.

The US Chamber of Commerce and other business voices, however, say NAFTA has greatly benefited the US economy in its 23 years of existence. Trade has tripled in that time, and withdrawal could badly hurt American farmers.

For Mexico, preserving the pact is vital, as 80 percent of its exports go to the United States.

Negotiations on changing the accord are expected to go on into early next year