UPDATE 1-U.S. did not raise auto rules of origin at NAFTA talks -source

(Adds details, background)

WASHINGTON, Aug 19 (Reuters) - In the opening NAFTA session of talks, the United States did not raise its previously stated demands to boost North American content for autos, a source directly familiar with the negotiations said on Saturday.

Robert Lighthizer, President Donald Trump's top trade adviser, this week said Washington wanted tougher rules of origin for autos, which determine how much of a vehicle must be built in the three NAFTA nations.

He also said the United States was seeking new measures to ensure "substantial U.S. content" for autos.

But during the opening four-hour round of talks on rules of origin on Friday, the U.S. delegation did not mention either demand, said the source, who asked not to be identified because of the sensitivity of the matter.

A spokesperson for Lighthizer's office did not immediately respond to a request for comment.

Companies wishing to take advantage of free trade in good guaranteed by NAFTA must currently meet the 62.5 percent North American content requirement for autos and 60 percent for components.

The United States, Canada and Mexico on Wednesday opened talks in Washington to modernize the North American Free Trade Agreement, which was signed in 1994. Trump has denounced NAFTA as a "disaster" that encouraged firms to shift production to Mexico.

Administration officials say strengthening the rules of origin for autos will help boost well-paid jobs in the United States as well as cut the trade deficit with Mexico, another key Trump goal.

Auto industry groups from Canada, Mexico and the United States are pushing back against the demand for higher U.S. automotive content, saying it would be too complex.

According to a schedule seen by Reuters, negotiators are due to continue discussing rules of origin on Saturday as well as Sunday morning.

(Reporting by Anthony Esposito and David Ljunggren; Editing by Lisa Von Ahn and Mary Milliken)