(Reuters) - U.S. burger chain operator Wendy's Co <WEN.O> said it had discovered additional instances of unusual credit card activity at some of its franchise-operated restaurants, widening the scope of an earlier cyber attack on the company.

The company in January said it was investigating reports of unusual activity with payment cards used at some of its restaurants.

Wendy's said it recently discovered a variant of a malware that was discovered and reported in May. The new malware was used to target a point-of-sales system that was earlier believed to be unaffected.

The company said the new variant of the malware had been disabled in cases where it was detected.

Wendy's expects the number of franchise restaurants that will be impacted by the cybersecurity attacks is now "considerably higher" than the 300 restaurants already affected.

"To date, there has been no indication in the ongoing investigation that any company-operated restaurants were impacted by this activity," Wendy's said on Thursday.

The new discoveries are a result of the company's continuing investigation into unusual credit card activity at its restaurants.

Large retailers such as Target Corp <TGT.N> and Home Depot Inc <HD.N> have been victims of security breaches in recent years.

(Reporting by Narottam Medhora in Bengaluru; Editing by Shounak Dasgupta)