US retail giant Sears Holding Corp is planning a "mass layoff" of at least 500 employees at its Chicago headquarters, according to a revised report filed with the state's Department of Commerce and Economic Opportunity.

Originally, Sears - which was formed in March through a $12.3 billion merger of Sears Roebuck and Co and Kmart Holding Corp, said it planned to axe 250 workers, but has now upped the figure to a minimum of 500.