Callers from the Indian centres posed as "phantom debt" collectors who targeted people who had applied for small online loans.

The Federal Trade Commission said two-California based firms and their owner had made $5m (£3.2m) from the fraud.

FTC officials said this was the first case of its kind as more than 10,000 people across the country were cheated.

"We think we have a really serious problem," said C Steven Baker, director of the Federal Trade Commission's Midwest Region, in Chicago.

"As economies have globalised, so has fraud."

A US court has frozen the assets of California-based firms American Credit Crunchers LLC, Ebeeze LLC and their owner, Varang K Thaker.

They have been charged with violating the FTC Act and the Fair Debt Collection Practices Act, though no criminal charges have been filed.

Mr Thaker allegedly withdrew thousand of dollars that had been paid into the companies' accounts by victims of the fraud.

Mr Baker said it was not clear if the operation was directed from California or India.

He said authorities had received more than 4,000 complaints about debt-collection schemes in recent years.

The callers used personal data on their victims obtained from payday loan websites - which usually extend small, short-term loans at high interest rates to borrowers.

More than 20 million calls may have been made over the past two years, Mr Baker said, with collectors using aggressive and threatening language to demand payment for debts that did not exist.

"Nobody actually owed them a dime," Mr Baker said.

David Vladeck, director of the FTC's consumer protection bureau, said: "Consumers should not be pressured into paying debt they don't remember owing. Legitimate debt collectors must provide consumers with both written information about the debt, and instructions for protecting themselves if they don't think they owe the debt."

(BBC Report)