Chicago Mercantile Exchange Holdings Inc. is planning to sweeten its bid for CBOT Holdings Inc. (BOT.N: Quote, Profile, Research), the Wall Street Journal reported on Thursday.

CME, the largest U.S. futures exchange, has been engaged in a bidding war for the parent of the No. 2 U.S. futures exchange with IntercontinentalExchange Inc. (ICE.N: Quote, Profile, Research) since March.

Atlanta-based energy exchange ICE raised its offer for CBOT on Tuesday, a day after the U.S. Department of Justice gave the regulatory clearance for CME and CBOT to merge.