(Reuters) – Burger chain operator Wendy’s Co said on Wednesday it was investigating reports of unusual activity with payment cards used at some of its 5,700 locations in the United States.

“Reports indicate fraudulent charges may have occurred elsewhere after payment cards were legitimately used at some restaurants,” Wendy’s spokesman Bob Bertini told Reuters in an email statement.

Large retailers such as Target Corp and Home Depot Inc have been victims of security breaches in recent years. Gourmet sandwich chain Jimmy John’s was also breached in 2014.

“Until this investigation is completed, it is difficult to determine with certainty the nature or scope of any potential incident,” Bertini said. “We have hired a cyber security firm to assist, but are not disclosing the name at this point.”

Security blog Krebs on Security first reported the development earlier in the day.

(Reporting by Subrat Patnaik and Sruthi Ramakrishnan in Bengaluru; Editing by Savio D’Souza and Maju Samuel)