(Bloomberg) -- Wendy’s Co., the third-biggest U.S. burgerchain, is investigating reports of unusual activity with paymentcards used at some of its 5,700 domestic locations.

“Reports indicate fraudulent charges may have occurred elsewhereafter payment cards were legitimately used at some restaurants,”Bob Bertini, a spokesman for the Dublin, Ohio- based company, saidin an e-mailed statement. “We have launched a comprehensiveinvestigation with the help of cybersecurity experts to gatherfacts, while working to protect our customers.”

Retailers and restaurants are increasingly becoming victims ofmajor hacker attacks. In December, Target Corp. said it would pay about $39million to banks and credit unions to resolve losses from a2013 holiday-season data breach. Other major incidents within thepast couple of years include hacks at Home Depot Inc. and auctionsite EBay Inc. In 2014, Jimmy John’s sandwich chain said a hackerinfiltrated payment systems at some of its restaurants.

Related: Cyber insurance 2015: Inside a robust and rapidlychanging market

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

“Until this investigation is completed, it is difficult todetermine with certainty the nature or scope of any potentialincident,” Bertini said.

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