NEW YORK (Reuters) -- Wal-Mart Stores Inc. has used a myriad of tactics, including some that are illegal, to hinder the ability of its workers to form labor unions, a human rights group said in a report to be released Tuesday.

According to Human Rights Watch, the world's largest retailer has restricted the dissemination and discussion of pro-union views, threatened to withhold benefits from workers who organize, interrogated workers about their union sympathies and sent managers to eavesdrop on employee conversations.

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Wal-Mart (Charts, Fortune 500), the largest private employer in the United States, has also refused to bargain collectively, fired employees it knows to be pro-union and focused security cameras on areas where union organizing is heaviest, according to the report.

Wal-Mart employs more than 1.3 million workers nationwide, none of which is in a union. "Wal-Mart workers have virtually no chance to organize because they're up against unfair U.S. labor laws and a giant company that will do just about anything to keep unions out," said Carol Pier, who researched the report for Human Rights Watch, which probes human rights abuses around the world.

Wal-Mart spokesman David Tovar said the Human Rights Watch report is based on "unsubstantiated allegations" and added that the retailer respects its workers' right to a free and fair unionization vote.

"Wal-Mart provides an environment of open communications and gives our associates every opportunity to express their ideas, comments and concerns," Tovar said in an emailed statement.

"It is because of our efforts to foster such an environment that our associates have repeatedly rejected unionization attempts."

Human Rights Watch, based in New York, said it interviewed 41 current and former Wal-Mart workers and managers, met with labor lawyers and union organizers, and analyzed lawsuits that accuse Wal-Mart of violating U.S. labor laws.

"Wal-Mart is a poster child for what is wrong with U.S. labor laws," said Pier. "Many tactics comport with U.S. law but taken together they create a climate of fear and intimidation."

Wal-Mart exposes new hires to anti-union training sessions and videos, gives managers union-prevention manuals and uses a "union hotline" and a centralized database to track union activity across the country, Human Rights Watch said.

U.S. labor laws fall short of international standards, the report said, and "allow a wide range of employer conduct that violates workers' right to organize and fail to include 'sufficiently dissuasive sanctions against acts of interference by employers against workers and workers' organizations,' as required by law."

Wal-Mart, which competes against Target (Charts, Fortune 500) and Costco (Charts, Fortune 500), operates some 4,000 stores nationwide, has battled critics who charge it with providing poverty-level wages and inadequate health care.