Citigroup Inc has acknowledged that a computer breach may have given hackers access to the data of hundreds of thousands of bank card customers, the Financial Times reported on Thursday.

The bank said it had discovered the breach in early May through routine monitoring, and it revealed details on Wednesday after being questioned by the Financial Times, the paper reported.

The bank said about 1 percent of its card customers were affected by a breach at Citi Account Online, which holds basic customer information such as names, account numbers and email addresses, the Financial Times said. Citi Cards has about 21 million customers in North America, according to Citi's annual report, the paper said.

Other information such as birth dates, social security numbers and card security codes is held elsewhere and was not compromised, the paper reported.

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

Citigroup was not immediately available for comment.

(Reporting by Abhishek Takle in Bangalore)