A US judge on Monday approved the suit against TCS, India's largest software outsourcer and part of the sprawling tea-to-steel Tata conglomerate.

The plaintiffs accuse the Mumbai-based company of forcing its Indian employees to sign over their US tax refunds, according to the Lieff Cabraser Heimann & Bernstein law company, which is representing them.

The complaint also alleges that TCS did not pay its employees the amount it promised them before going to the United States.

"This is an order only on one procedural matter and does not address the merits of this case," a TCS spokesman said in an emailed response to AFP, confirming receipt of the order without giving further details.

"We continue to believe that when this matter concludes, the court will find that the plaintiff's claims are without any merit," the statement said.

The ruling allows all non-US citizens employed in the United States between February 14, 2002 and June 30, 2005, and those sent to the United States after January 1, 2002, to take part in the lawsuit.

"More than 10,000 current and former Indian nationals working for Tata in America now may have their day in court," Kelly Dermody, co-lead class counsel, said in a statement.

"We look forward to demonstrating at trial that Tata breached the standard employment contract with these employees and violated California labour laws."

The plaintiffs seek "compensation and damages for current and former employees who were not paid what they were promised, who were deprived of their tax refunds, and who faced unauthorised Indian salary deductions," the law firm said on its website.

TCS employs about 214,000 people in 42 countries, and had revenue of $8.2 billion in the fiscal year ending March 31, 2011, according to its website.

The move came after Yahoo! filed suit against Facebook in a California court last month accusing the social networking giant of infringing on 10 patents.

"From the outset, we said we would defend ourselves vigorously against Yahoo!'s lawsuit, and today we filed our answer as well as counter-claims against Yahoo! for infringing 10 of Facebook's patents," said Facebook general counsel Ted Ullyot.

"While we are asserting patent claims of our own, we do so in response to Yahoo!'s short-sighted decision to attack one of its partners and prioritize litigation over innovation."

Yahoo!, in a 19-page lawsuit filed in US District Court for the Northern District of California, accused Facebook of infringing on patents in several areas including advertising, privacy and messaging.

In the suit, Yahoo! said Facebook's growth to some 850 million users "has been based in large part on Facebook's use of Yahoo!'s patented technology."

The Sunnyvale, California-based Yahoo! asked the court to order Facebook to halt its alleged patent-infringing activities and to assess unspecified damages.

Yahoo! did not respond to AFP requests for comment.

But its lawsuit was filed as Facebook was poised to go public on Wall Street, perhaps as soon as May, with a valuation of up to $100 billion.

Facebook recently amended paperwork filed with the US Securities and Exchange Commission to warn potential investors that Yahoo!'s patent lawsuit could deliver a significant blow to its business.

"This litigation is still in its early stages and the final outcome, including our liability, if any, with respect to these claims, is uncertain," Facebook said in the updated SEC filing.

"If an unfavorable outcome were to occur in this litigation, the impact could be material to our business, financial condition, or results of operations."

Patent suits are a frequent occurrence among smartphone and tablet computer makers, and the world's best known brands are ensnared in a complex web of legal claims, but such suits are relatively rare among social media companies.

The countersuit rejected a claim that "without Yahoo!'s achievements websites such as Facebook would not enjoy repeat visitors or substantial advertising revenue."

Facebook, which is shifting operations to a former Sun Microsystems campus in the Northern California city of Menlo Park, denied violating any valid Yahoo! patents.

Facebook went on to accuse Yahoo! of infringing on its patented technology in a broad array of products including online venues for news, games, cars, travel, and photo-sharing service Flickr.

Facebook asked the court to dismiss Yahoo!'s complaint entirely and make the Internet veteran pay damages as well as the social network's legal costs.