Wendy’s Co., the third-biggest U.S. burger chain, is investigating reports of unusual activity with payment cards used at some of its 5,700 domestic locations.

“Reports indicate fraudulent charges may have occurred elsewhere after payment cards were legitimately used at some restaurants,” Bob Bertini, a spokesman for the Dublin, Ohio- based company, said in an e-mailed statement. “We have launched a comprehensive investigation with the help of cybersecurity experts to gather facts, while working to protect our customers.”

Retailers and restaurants are increasingly becoming victims of major hacker attacks. In December, Target Corp. said it would pay about $39 million to banks and credit unions to resolve losses from a 2013 holiday-season data breach. Other major incidents within the past couple of years include hacks at Home Depot Inc. and auction site EBay Inc. In 2014, Jimmy John’s sandwich chain said a hacker infiltrated payment systems at some of its restaurants.

Fast-food chains are competing fiercely to draw diners with their deals. Wendy’s, which has recently advertised a 4-for-$4 promotion, said in November that same-store sales increased 3.1 percent at its North American locations in the third quarter.

“Until this investigation is completed, it is difficult to determine with certainty the nature or scope of any potential incident,” Bertini said.

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