(Changes sourcing, new throughout)

Sept 26 (Reuters) - U.S. fast-food chain operator Sonic Corp said on Tuesday it had been notified of unusual activity regarding credit cards used at its drive-in outlets.

Sonic's credit card processor informed the company earlier this week of the activity, which could have affected an unknown number of its payment systems.

Cyber security blog KrebsOnSecurity first reported the news and added that the activity may have led to millions of stolen credit and debit card numbers being sold in underground exchanges. (http://bit.ly/2xve25F) (Reporting by Ahmed Farhatha in Bengaluru; Editing by Sandra Maler)