June 3, 2014 - American Express is writing to around 75,000 Californian customers to warn them that their card data has been posted online by Anonymous Ukraine.

In March the Ukrainian arm of the Anonymous collective posted details from around seven million Visa, MasterCard, AmEx and Discover cards online in protest at the US and its banking system.

While the fraud threat is considered minimal, AmEx has now moved to warn affected customers. As picked up by CSO, the firm has written to the California Attorney General's Office about its decision.

AmEx is writing to 58,522 Californians whose names and corresponding account numbers were involved, as well as another 18,086 whose card information was published but not their names.

Says the letter being sent out: "We were recently made aware that your American Express Card information was recovered during an investigation by law enforcement and/or American Express.

"At this time, we believe the recovered data may include your American Express Card account number, the card expiration date, the date your card became effective and the four digit code printed on the front of your card."

The company stresses that social security numbers were not taken and that no fraud has been detected on the affected cards but says that it has put extra monitoring procedures in place.