(Yicai Global) Dec. 3 -- A data breach at American hotel chain Marriott International may have compromised information on up to 500 million of its guests.

The Maryland-based firm first received an internal security alert to the hack in September, and discovered the an unauthorized party has been accessing the data since 2014, copying, encrypting and attempting to delete certain parts of information, the world's largest hotel chain said on social media on Nov. 30. The data concerns guests of its Starwood Hotels brand.

Marriott has filed a report with police.

"The company has not finished identifying duplicate information in the database, but believes it contains information on up to approximately 500 million guests who made a reservation at a Starwood property," the firm said in a statement.

For around 327 million of the guests, the information includes some combination of the following datasets: name, address, phone number, email address, passport number, Starwood Preferred Guest account data, date of birth, gender, reservation dates, travel information and communication preferences, it added.

For some, payment card information may have been obtained -- but it is encrypted by 128 bit Advanced Encryption Standard, Marriott said, adding that it does not know whether or not the hackers have cracked the two security keys.

Editor: James Boynton