Hackers breached Citibank’s network and viewed details of roughly 200,000 North American bank card holders, the US bank told the Financial Times on Wednesday.

The bank said that names, account numbers and contact details for roughly 1% of Citi Account Online customers were visable in the breach, which was discovered in early May during routine testing. However, Citi said the attack did not compromise all user details and that birth dates, card expiration dates and CCV codes were not disclosed.

U.S.-based Citi spokesman Sean Kevelighan told Reuters that the bank was contacting the impacted customers. "Citi has implemented enhanced procedures to prevent a recurrence of this type of event," he said. "For the security of these customers, we are not disclosing further details."

It is not the first time Citibank has suffered security issues. Last year, the US bank warned users of its iPhone-based banking application that a bug may have leaked personal details including bank account numbers.

In 2009, the Wall Street Journal reported that hackers, supposedly Russian in origin, had stolen millions of dollars from the bank. However, Citibank denied all knowledge of the incident.

And back in 1995, Citibank was one of the first financial institutions to be targetted by cyber criminals. Hacker Vladimir Levin was charged for using stolen customer details to steal $10 million. Interestingly, Levin used the company’s phone network, not the Internet, to access payment systems.