In a statement to security blogger Brian Krebs, spokesman Bob Bertini says: "We have received this month from our payment industry contacts reports of unusual activity involving payment cards at some of our restaurant locations.

"Reports indicate that fraudulent charges may have occurred elsewhere after the cards were legitimately used at some of our restaurants. We’ve hired a cybersecurity firm and launched a comprehensive and active investigation that’s underway to try to determine the facts."

The fraudulent activity appears to have occurred late last year, although Wendy's says it is too soon to know whether the situation is now contained or how many of its 5700 US sites have been affected.