American retailer Target is reportedly investigating a data breach that affected the in-store records of millions of credit and debit card transactions.

According to multiple sources, the breach is expected to have started on Black Friday 2013, which is the busiest shopping day of the year.

The data breach was reported at all the Target locations in the nation and involved theft of data that is stored on the magnetic stripe of cards used at stores.

According to media reports, the theft of data was initially confined to period between Thanksgiving 2013 and 6 December, however, investigations have found that the breach was extended till 15 December 2013.

A top ten US bank card issuer anti-fraud analyst said the breach window is definitely expanding.

"We can’t say for sure that all stores were impacted, but we do see customers all over the U.S. that were victimized," analyst added.

Another source said it appears that when all is said and done, this one will put its mark up there with some of the largest retail breaches to date.