EXCLUSIVE-CBS plans all-stock bid for Viacom below current valuation -sources

(Adds details about merger talks, company background)

April 2 (Reuters) - CBS Corp plans to make an all-stock offer for Viacom Inc that values the U.S. media company below its current market valuation, people familiar with the matter said on Monday, indicating that tough negotiations lie ahead.

It is unusual for deal negotiations to start with the acquirer valuing its target at a discount. The fact that CBS's first bid for Viacom infers such a valuation reflects how CBS views its position in the U.S. media landscape as superior to Viacom's.

As part of its initial offer to be submitted in the next few days, CBS Chief Executive Leslie Moonves will propose to stay at the helm for at least two years if the U.S. media company's bid to merge with peer Viacom is accepted, according to the sources.

That offer is contingent on Viacom accepting all other aspects of CBS's bid, the sources added.

The persons asked not to be identified because the matter is confidential.

CBS and Viacom declined to comment.

CBS and Viacom are exploring a merger at the urging of their common controlling shareholder, National Amusements Inc, the family holding company of Sumner Redstone and his daughter Shari.

The two companies explored a merger in 2016 instigated by National Amusements, but those talks ended unsuccessfully, due to concerns by CBS about price and governance issues.

Since then, Viacom CEO Bob Bakish has taken a number of steps to boost the company's performance, including improving relations with cable and satellite companies, causing some analysts to expect CBS to acquire Viacom at a premium.

(Reporting by Jessica Toonkel in New York Editing by Matthew Lewis)