Mexico City - Mexican officials warned last week that U.S. proposals in ongoing NAFTA negotiations would result in protectionism and higher prices rather than free trade, as concerns about the future of the deal helped drag the peso to four-month lows.

Negotiators from Canada, Mexico and the United States are due to begin a fourth round of talks to renegotiate the North American Free Trade Agreement in Washington this week.

Senators from President Enrique Peña Nieto's Institutional Revolutionary Party (PRI) said they would oppose any deal that included proposals such as setting a national quota for U.S.-made auto parts and increasing controls over the trade in fresh produce, people close to the talks said.

"We are deeply worried by what the U.S. has added recently in the free-trade negotiations," said Senator Marcela Guerra, who heads Mexico's Senate committee on North American affairs.

U.S. President Donald Trump argues that NAFTA has hollowed out U.S. manufacturing, sending jobs to Mexico and fueling a U.S. goods trade deficit of more than $60 billion with its southern neighbor.

Senator Ricardo Urzua said such proposals would hurt consumers who have enjoyed lower prices under NAFTA. "Far from trying to favor certain U.S. labor sectors, this would hurt millions of consumers in the United States, Canada and Mexico," Urzua said.

A lack of substantive progress in three rounds of NAFTA talks since August and concerns about the hardline U.S. positions has added to market jitters in recent days. The peso currency closed down 1.29 percent at 18.4995 per dollar on October 5, the lowest since June 5.