The U.S. Secret Service is investigating a major cyber intrusion at an Atlanta-based payment processor that could expose millions of MasterCard, Visa, American Express and Discover cardholders to fraudulent charges, Reuters reported.

Processor Global Payments Inc said on Friday, March 30 it had found "unauthorized access" into its system early in March and notified law enforcement and financial institutions.

Payment network operators MasterCard Inc, Visa Inc, American Express Co and Discover Financial Services confirmed they were affected, along with banks and other franchises that issue cards bearing their logos.

A spokesman for the Secret Service said the agency is leading investigations into the case but declined to give any details.

Though Global Payments is far from a household name, middlemen such as the company are prized targets for hackers because of the vast amount of sensitive financial information they handle.

The company's stock fell more than 9 percent on the news before trading was halted. It said it would discuss the breach in a phone call for investors on Monday.

It was not immediately clear how Global Payments was penetrated or how many accounts were exposed. Consumers who detect fraud usually can be reimbursed. That leaves merchants on the hook financially, though they could file claims against Global Payments.

JPMorgan Chase & Co, as well as American Express and Discover, which issue their own cards, said they are monitoring customers' accounts and would issue new cards to anyone whose information may have been compromised.

Citigroup Inc said it has been notified by processors of the breach. Bank of America Corp declined to comment on the matter and Wells Fargo & Co said it was too early to comment on the impact.

Banks and processors emphasized customers would not be held liable for any fraudulent charges that may occur.