US grocery retailer Kroger has revealed that federal prosecutors are investigating whether it hired locked-out employees during a strike at its Ralphs grocery chain in Southern California.

Kroger said the Los Angeles office of the US Attorney had convened a grand jury to probe claims that some Ralphs stores hired locked-out employees who were using false names and social security numbers, reported Reuters.

The company said it was cooperating with the investigation and did not know whether it would face charges or what potential penalties any charges would incur.

Around 60,000 grocery workers were either on strike or locked out from October until the end of February in Southern California branches of Ralphs, Safeway and Albertsons, in a dispute that centred on health care costs.