Wendy’s said Wednesday that it was investigating reports of “unusual activity” on credit cards that were used at some of its locations, the Associated Press reported. The fast food company said payment industry contacts informed it this month of the possible fraudulent charges.

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

The company advised that customers should keep an eye out for possibly unauthorized charges on debit or credit cards if they've visited a Wendy's location in recent months.

The company said it remained unclear how long the issue has persisted or whether the incident was yet contained, KrebsOnSecurity reported. The security news website said it initially heard about a possible breach at Wendy’s branches from banks in the Midwest but has more recently heard reports from financial institutions on the East Coast.

“We began investigating immediately, and the period of time we’re looking at the incidents is late last year,” Bob Bertini, a spokesman for Wendy's, said. “We know it’s [affecting] some restaurants, but it’s not appropriate just yet to speculate on anything in terms of scope.”

Wendy’s, founded in in 1969, includes nearly 6,500 franchise and company-managed restaurants in at least 28 countries and U.S. territories worldwide.

Hackings have occurred at other large chains. Last year, Target agreed to pay $10 million to settle a class action related to a data breach in 2013. Some 42 million people were estimated to have had their credit or debit card information stolen, according to court documents. The proposed settlement required Target to promptly change its security policies.