Citigroup confirmed on Thursday that unidentified hackers breached security and have viewed some of its credit-card customers' account information in the U.S.

The bank said the breach exposed information like names, account numbers and email addresses, while other information such as security numbers, birth dates, card expiration dates and CVV codes are kept elsewhere and not compromised.

"We are contacting customers whose information was impacted. Citi has implemented enhanced procedures to prevent a recurrence of this type of event," a Citi spokesman told Reuters.

“During routine monitoring, we recently discovered unauthorized access to Citi’s Account Online. A limited number roughly one percent of Citi bankcard customers’ account information was viewed,” the bank said.

The New York-headquartered bank said about one percent of its card holders had been affected but didn't estimate the actual number affected. According to 2010 annual report, the bank has more than 21 million credit card customers in US.

Citigroup is the latest firm to be hacked in recent weeks after Sony, which has been hit by a series of hacks in the recent months.

Sony, which is still reeling from a hacking attempt in April, was hit by another massive data breach last week.

A hacker group called Lulz Security, the master hackers who attacked the PBS website and posted a fake story about rapper Tupac Shakur being alive, claimed credit.

As proof for hacking Sony, LulzSec has published Sony’s alleged compromised data on www.mediafire.com. The unzipped folder contains 4.66 MB of information, including what is claimed to be emails and passwords of Sony’s The Young and the Restless users.