The group, which included people from Estonia, Russia and Moldova, was accused of compromising the data encryption used by RBS WorldPay, based in Atlanta and part of Royal Bank of Scotland, and gaining access to some accounts.

"This investigation has broken the back of one of the most sophisticated computer-hacking rings in the world," said Sally Quillian Yates, the acting U.S. attorney for northern Georgia.

RBS WorldPay is one of the leading payment processing businesses globally.

The individuals were indicted by a grand jury in Atlanta, and Estonian police have arrested four of them, the Justice Department said. One of the accused, Sergei Tsurikov of Estonia, already is awaiting extradition to the United States.

The Justice Department identified the accused as five Estonians, one Russian, one Moldovan and one individual whose nationality was not specified.

The ring was charged with hacking data for payroll debit cards, which enable employees to withdraw their salaries from automated teller machines. Those accused in the case allegedly raised the limits on some cards so they could withdraw the money, the U.S. government said.

More than $9 million (5.4 million pounds) was withdrawn in less than 12 hours from more than 2,100 ATMs around the world, the Justice Department said, adding that RBS WorldPay immediately reported the breach once it was discovered.

RBS WorldPay is part of global merchant services, one of the assets Royal Bank of Scotland is being forced to sell to satisfy EU antitrust concerns over state aid it has received.

RBS, which will be 84 percent state-owned after a fresh bail-out deal agreed last week, is one of the top recipients of state aid. It has up to four years to sell the assets.

(Reporting by Jeremy Pelofsky and Clara Ferreira Marques in London, editing by Will Dunham)