Mexico, the United States and Canada have begun the fifth round of talks on renegotiating the North American Free Trade Agreement.

None of the three countries' trade representatives or economy secretaries will directly attend the talks, which will formally open Friday and run through Tuesday. But the U.S Trade Representative's office said Wednesday that 30 groups of lower-level negotiators are meeting in Mexico City this week.

Talks involving upper-level officials were held this month at the Asia-Pacific APEC meetings in Vietnam.

Talks have stalled over tough American demands, including higher U.S. content for automobiles, changes to the dispute-resolution process and a sunset clause that could force the renewal of the pact every five years.

Mexico's low wages have also sparked demands it implement better labor protections.