NEW YORK - Target says up to 40 million card accounts may have been affected by data breach during the height of the holiday shopping season.

Target announced this morning that it believes the breach impacted cards used by shoppers between Nov. 27 and Dec. 15. The company says it “alerted authorities and financial institutions immediately after it was made aware of the unauthorized access, and is putting all appropriate resources behind these efforts.”

The Secret Service says it is working with other law enforcement agencies on the investigation.

The breach was first reported by the popular security blog, KrebsOnSecurity.

Minneapolis-based Target Corp. has 1,797 stores in the U.S. and 124 in Canada. The company says anyone who suspects unauthorized activity should contact Target at: 866-852-8680.