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 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 breach affected all cards, including Target store brand cards and major card brands such as Visa and MasterCard.

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 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 see suspicious charges on the 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.”

But news of the breach comes at the height of the critical holiday shopping season and threatens to scare away shoppers worried about the safety of their personal data. The November and December period accounts for 20 percent, on average, of total retail industry sales.