Target says that about 40 million credit and debit card accounts may have been affected by a data breach that occurred just as the holiday shopping season shifted into high gear.
The chain said that customers who made purchases using their cards at its US stores between November 27 and December 15 may have been exposed.
The Minneapolis company said it immediately told authorities and financial institutions once it became aware of the breach and that it's teaming with a third-party forensics firm to investigate the matter.
It said it is putting all "appropriate resources" toward the issue.
Target Corp advised customers who suspect there has been unauthorised activity on their cards to call them.
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"Target's first priority is preserving the trust of our guests and we have moved swiftly to address this issue, so guests can shop with confidence. We regret any inconvenience this may cause," Chairman, President and CEO Gregg Steinhafel said in a statement on Thursday.
Target is the latest retailer to be hit with a data breach problem.
TJX Cos, which runs stores such as T.J. Maxx and Marshall's, had a breach that began in July 2005 that exposed at least 45.7 million credit and debit cards to possible fraud.
The breach wasn't detected until December 2006.
In June 2009, TJX agreed to pay $US9.75 million in a settlement with multiple states related to the massive data theft, but stressed at the time that it firmly believed it did not violate any consumer protection or data security laws.
Target has 1797 US stores and 124 in Canada.
