Neiman Marcus said Friday that some customers' credit cards were compromised in a security breach, but it provided scant detail on the scope of the hack.
The data attack on the upscale department store chain was revealed on the same day that big-box retailer Target announced the number of its customers affected by a breach had swelled from 40 million to 70 million.
(Read more: Feeling vulnerable? Target shoppers can protect their info)
Neiman Marcus did not say how many of its customers might have been victimized by a "criminal cyber-security instrusion" it confirmed Jan. 1 — up to two weeks after an alarm was raised by its credit-card processor.
"Neiman Marcus was informed by our credit card processor in mid-December of potentially unauthorized payment card activity that occurred following customer purchases at our Neiman Marcus Group stores," the company said in a statement.
(Read more: Target: Data breach affected up to 110 million)
A forensics firm retained to investigate the situation confirmed the hack and found "that some customers' cards were possibly compromised as a result."
"We have begun to contain the intrusion and have taken significant steps to further enhance information security," the statement said.
Neiman Marcus said it is working with the U.S. Secret Service and trying to notify customers "whose cards we know were used fraudulently after making a purchase at our store."
Spokeswoman Ginger Reeder said she could not provide any other information, including how many cards may have been affected and if it was a nationwide issue.
Major card issuers continued to monitor the situation and reassured affected customers that they are protected in the event of theft.
"We are monitoring accounts for suspicious activity," a spokesperson for Citigroup told CNBC. "Our customers are not liable for any unauthorized charges."
A Bank of America spokesperson also said it's customers are covered in the event of theft. "We always proactively monitor customers' accounts for fraud. If we believe the account is at risk for fraud, we will notify a customer and reissue the card," the spokesperson told CNBC. "We also encourage customers to monitor their accounts and let us know immediately if they have fraudulent transactions on their accounts."
American Express urged customers to update their profiles to include mobile phone numbers. "This way, if we do see anything that we suspect as fraudulent, we can be in touch with the card member as soon as possible, via mobile," a company spokesperson told CNBC.
—By Tracy Connor of NBC News. CNBC contributed to this report.