MEXICO CITY, Oct 27 (Reuters) - The U.S. Trade Representative has declined to file a complaint with Mexico over alleged worker rights abuses at a Saint-Gobain glass plant, saying on Thursday that the factory's situation improved after workers elected an independent union.

The biggest U.S. labor federation last month asked the U.S. government to open a complaint, the latest in a series of petitions under the United States-Mexico-Canada Agreement (USMCA) trade pact alleging rights violations in Mexico.

A central plank of USMCA, a deal crafted at the behest of the United States, was that it would improve the lot of workers in Mexico and help stem the loss of U.S. manufacturing jobs to its lower-cost neighbor.

Major U.S. unions the AFL-CIO and United Steelworkers accused the Mexican arm of France's Saint-Gobain in the central state of Morelos of unfairly supporting its long-time union over a new, independent rival.

The office of the U.S. Trade Representative $(USTR)$ called the allegations "troubling," but said that during its initial review, workers managed to elect the new union - the Independent Union of Free and Democratic Workers of Saint-Gobain Mexico - which had also signed the petition.

"The situation at the facility improved for workers," USTR said in a statement, adding that Saint-Gobain had issued a statement recognizing the new union.

"Further immediate action on the petition is not needed at this time," the USTR said. U.S. and Mexican officials will monitor the situation, it noted.

Under the new union, workers can now seek better wages and conditions in line with USMCA's goals, U.S. Labor Secretary Marty Walsh said in the statement.

Saint-Gobain did not immediately respond to a request for comment.

AFL-CIO International Director Cathy Feingold said the federation would check that Saint-Gobain bargains in good faith with the union for a new collective contract, and urged the United States to also track the matter.

Since last year, U.S. officials have raised five other cases under USMCA, which has helped revitalize Mexico's labor movement.

(Reporting by Daina Beth Solomon Editing by Bill Berkrot)

((daina.solomon@thomsonreuters.com; +52 55 5282 7150;))