Target says about 40 million credit and debit card accounts may have been impacted by a data breach that occurred just as the holiday shopping season shifted into high gear.

The chain said that accounts of customers who made purchases by swiping their cards at terminals in its U.S. stores between Nov. 27 and Dec. 15 may have been exposed. The stolen data includes customer names, credit and debit card numbers, card expiration dates and the three-digit security codes located on the backs of cards. The data breach did not affect online purchases.

The Minneapolis company said it immediately told authorities and financial institutions once it became aware of the breach and that it is teaming with a third-party forensics firm to investigate the matter and prevent future breaches. It said it is putting all "appropriate resources" toward the issue.

Target Corp. advised customers to check their statements carefully. Those who suspect there has been unauthorized activity on their cards should report it to their credit card companies and call Target at 866-852-8680. Cases of identity theft can also be reported to law enforcement or the Federal Trade Commission.

Target didn't say exactly how the data breach occurred, but said it had since fixed the problem and that credit card holders can continue shopping at its stores. When asked whether there's a certain time when shoppers know their accounts will no longer be vulnerable, a Target spokeswoman said, "We encourage everyone to be vigilant."

Nicole D'Auria, of Selden, was one of millions of Target customers affected by the data breach, right at the height of the holiday shopping season.

"I went onto the site, and I looked in it and it was literally Target in the Bronx, and it was one transaction, and then another one five minutes later and then another one five minutes later after that," she said.

D'Auria says that about $700 was taken from her, which was all of the money in her bank account. She says her money has since been refunded, but she is now using cash when she shops in the store.

Financial advisor Jon Ten Haagen says the best way to protect oneself is to check accounts daily. If a consumer suspects hacking, they should take immediate action. This includes getting a new card or a new PIN. It's important to change all automatic payments related to that account.

Target has 1,797 U.S. stores and 124 in Canada.

AP Wire Services contributed to this report