Computer hackers have breached Citigroup's computer network and have accessed data on hundreds of thousands of its card customers, the Financial Times said.

Citigroup said the breach, which affected about 1 per cent of its card customers, was discovered in early May through routine monitoring, the newspaper said.

According to the bank's annual report, Citi Cards has about 21 million customers in North America.

The breach occurred at Citi Account Online, which holds basic customer information such as names, account numbers and e-mail addresses.

Other information such as birth dates, social security numbers and card security codes are held elsewhere and were not compromised, Citi said.

"The bank said it had contacted law enforcement officials and tightened its fraud detection procedures, but declined to provide further details or to say whether customers had reported suspicious transactions," the FT reported.

Though Citigroup said the breach involved only credit card accounts, the FT said that several people have told it their debit card details were compromised.

Hacking into companies is increasingly becoming common.

Lockheed Martin, PBS and Sony have all recently had their security systems violated.