Payroll firm Automatic Data Processing Inc said tax and salary data of employees of "a small number" of its clients had been stolen.

The company said on Tuesday that the theft occurred because some of its clients mistakenly published unique access codes to employee accounts online.

"The combination of an unsecured company registration code and stolen personal information enabled the fraudulent access to the portal," ADP said in a statement.

ADP also said it was working with a federal law agency regarding investigation into the data theft.