UPDATE 1-U.S. has not moved an inch during the NAFTA talks, say sources

(Adds quote from source, details, background)

MONTREAL, Jan 25 (Reuters) - U.S. negotiators have held firm in their demands for a wide-ranging overhaul of NAFTA, three sources close to the talks said on Thursday, raising questions about whether any real movement is happening at the latest round of negotiatins on the treaty.

Officials from Canada, Mexico and the United States are in Montreal for the sixth and penultimate set of talks on the North American Free Trade Agreement. Major differences remain to be settled ahead of the end-March deadline.

"We have brought flexibility, we have brought ideas, but the problem is that the United States has not moved an inch. They say, 'It is my proposal or nothing,"' said one of the sources, who spoke to reporters on condition they not be named.

The administration of U.S. President Donald Trump, which has repeatedly threatened to walk away from the 1994 pact, wants more North American content in autos and is pressing for a sunset clause that would allow one party to pull out of the treaty after five years.

The source cited news media reports indicating that the United States wanted to stay in the negotiations and was prepared to accept changes to its demands.

"Let's see if that is reflected in the (talks) and what the United States will bring. If they do (show flexibility), I think we are in a position to have quite important agreements," said the source. (Reporting by Anthony Esposito and David Ljunggren; Editing by David Gregorio and Jonathan Oatis)