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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.

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.

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.