BOSTON (Reuters) - A computer hacker stole thousands of credit card numbers after breaching security at two U.S. grocery store chains owned by Belgium-based Delhaize Group SA, the companies said on Monday.

Nearly 2,000 cases of fraud have been linked to the breach, but no personal information such as names or addresses was accessed when the hacker broke into the Hannaford Bros. stores in Massachusetts, New England and New York, and Sweetbay customers in Florida, Hannaford said in a statement.

Boston's WBZ radio said 4.2 million credit and debit card numbers were stolen. Company officials were not immediately available to confirm the number of stolen card numbers.

Hannaford, headquartered in Scarborough, Maine, said it became aware of unusual credit card activity on February 27 and began an investigation. It said the data was illegally accessed during the credit card authorization process.

Hannaford Chief Executive Ron Hodge offered an apology for the intrusion. There are 165 Hannaford stores in the U.S. Northeast and 106 Sweetbay supermarkets in Florida.

"We sincerely regret any concern or inconvenience this has caused," Hodge said in a statement. "We have taken aggressive steps to augment our network security capabilities."

The breach is the latest at a big U.S. retailer and comes after U.S. retail group TJX Cos Inc disclosed last year that data from 45.7 million credit and debit cards were stolen by hackers over a period of 18 months, as well as personal information for 451,000 people.

A group of banks later asserted in court documents that the number of consumer accounts were affected was closer to 94 million, a charge Massachusetts-based TJX denied.