HOUSTON (Reuters) - Royal Dutch Shell Plc (RDSa.L) and U.S. private equity firm Blackstone Group LP (BX.N) are working on a joint $10 billion bid for BHP Billiton Plc’s (BLT.L) U.S. shale assets, Sky News said on Thursday.

Shell and Blackstone’s offer will only be one of several credible proposals that BHP will receive for its U.S. shale operations, Sky News said, citing banking sources.

BHP, Shell and Blackstone declined to comment.