Citigroup said a cyber attack in May affected over 360,000 of its customers, or almost twice the initial number that the bank's figures had suggested.

After a week of being pressured over its delayed disclosure of the data breach, the third largest US bank by assets revealed more details in a statement late on Wednesday.

The breach, which Citigroup first made public on June 8, was initially detected on May 10. A total of 360,083 North American Citigroup credit card accounts were affected by the breach, the bank said.

The bank said about 1 percent of North American card customers had been affected, reiterating comments made last week. But the bank's annual report says that number of customers totals 21 million, which initially indicated that about 200,000, rather than over 360,000, customers had been affected.

Citigroup said it had reissued credit cards to 217,657 accounts along with a notification letter, Citi said. - Reuters