The NAFTA text, either bilateral or trilateral, is due by late Sunday night to meet US congressional notification requirements to allow Trump and outgoing Mexican President Nieto to sign the pact before Lopez Obrador takes office on Dec. 1.
After more than a year of talks, Mexico and the United States announced a bilateral deal on Monday, clearing the way for Canada to rejoin talks to update 24-year-old NAFTA, which accounts for over $1 trillion in annual trade between the three nations.
The Mexico-US discussions have focused on crafting new rules for the automotive industry, which US President Donald Trump has put at the heart of his drive to rework the 24-year-old pact he says has been a "disaster" for American workers.
Lopez Obrador, a leftist who won Mexico's July 1 presidential election in a landslide said the incoming administration's aim was to "start a new chapter in the relationship between Mexico and the United States, based on mutual respect."
Canada agreed with the United States on Thursday that slow-moving talks to update NAFTA should continue although they did not set a date for the next round, a senior official said in remarks casting further doubt on the chances of a deal this year.