NEW YORK - Potential victims of credit card fraud tied to Target's security breach said they had trouble contacting the discounter through its website and call centers.

Angry Target customers expressed their displeasure in comments on the company's Facebook page. Some even threatened to stop shopping at the store.

Target apologized on Facebook. It also said it's working hard to resolve the problem and is adding more workers to field calls and help solve website issues. And the discounter began offering 10 percent off for customers who shop on Saturday and Sunday and free credit monitoring services to those who've been affected by the issue.

The Minneapolis-based discounter said that while it's only heard of "very few" reports of fraud, it's reaching out to customers who made purchases by swiping their cards when the scam occurred. The company also said it's continuing its investigation into the matter.

"We take this crime seriously," said Target CEO Gregg Steinhafel, in a statement issued Friday.

Target's statements come after the nation's second-largest retailer acknowledged Thursday that data connected to about 40 million credit and debit card accounts was stolen as part of a breach that began over the Thanksgiving weekend.

The theft is the second-largest credit card breach in U.S. history, exceeded only by a scam that began in 2005 involving retailer TJX Cos. That incident affected at least 45.7 million card users.

Target advised customers to check their statements carefully. Those who see suspicious charges should report them to their credit card companies and call Target at 866-852-8680. Cases of identity theft can also be reported to law enforcement or the Federal Trade Commission.