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Credit card data from customers at at least 20 Hyatt, Sheraton, Marriott, and Westin hotels may have been compromised by hackers.

Hotels in more than 10 states including California, Florida, Minnesota, and Virginia may have been targeted by hackers who put malware into place to collect names, card account numbers, card expiration dates, and verification codes from early December through late June, said hotel operator HEI Hotels & Resorts. In some locations, information may have been collected going back as far as March 2015. 

Data was collected from cards used at restaurants, bars, spas, lobby shops and other facilities, HEI spokesman Chris Daly told Reuters. 

"We are treating this matter as a top priority, and took steps to address and contain this incident promptly after it was discovered," said HEI in a press release, adding that it is now safe for customers to use their credit cards at all hotel locations. 

HEI said it first discovered the breach when it transitioned payment card processing to a standalone system separate from the rest of its network. An investigation is underway as the company works with law enforcement, banks, and payment card companies to determine exactly what happened. 

It's not known yet precisely how many customers may have had their data stolen, Mr. Daly told PCWorld. 

"Due to guests paying in multiple outlets during a stay or even visiting multiple times, or visiting multiple locations managed by HEI, an exact number is difficult to calculate," he said. "Furthermore, HEI does not store credit card details."

HEI recommends that any customers who used a card at the affected properties review their account statements, keeping an eye out for unusual activity, over the past several months and in the future. If you believe your information may have been compromised, contact your credit or debit card company immediately. 

What should you do if your credit card info is stolen? Angela Colley of Dealnews writes: