The announcement by the iconic US chain about "recent unauthorized activity on our network" comes amid a wave of cyberattacks worldwide targeting everything from meatpacking plants to pipelines to public utilities, some of whom have had to pay ransoms to hackers.

"While we were able to close off access quickly after identification, our investigation has determined that a small number of files were accessed, some of which contained personal data," McDonald's said in a statement.

However, the files accessed did not contain information about customers' payments, nor were operations disrupted, the company said.

"Based on our investigation, only Korea and Taiwan had customer personal data accessed, and they will be taking steps to notify regulators and customers listed in these files."

The company said it had worked with "experienced third parties" to investigate the breach, but did not provide details on the scale of the hack.

"A few additional markets will take steps to address files that contained employee personal data," McDonald's said, without elaborating.