Viacom Inc.'s six independent directors will fight any attempt to oust them from the media conglomerate's 11-member board, director Fred Salerno said in a letter to shareholders sent Monday.

Salerno said the independent directors are facing a removal attempt "as a result of a chain of actions said to have been legally put in motion by the controlling shareholder of Viacom, Sumner Redstone."

The directors find the assertion "inexplicable" that Redstone was "acting of his own free will and with the mental competency to do so," Salerno said.

He added that they intend to continue with plans to "explore strategic options that might include a minority investment in Paramount," Viacom's movie studio.

The letter came three days after a statement from Redstone, issued by his spokesman, that said the 93-year-old media mogul was considering ousting Viacom Chief Executive Philippe Dauman and the board of directors..

Redstone, who holds 80 percent of the voting shares in Viacom and CBS Corp., earlier this month removed Dauman and Viacom board member George Abrams from the seven-person trust that will control the shares after Redstone exits.

Dauman, 62, has filed a legal challenge to stop his removal from the trust, arguing that Redstone was being manipulated by his daughter, Shari. She has called that allegation "absurd" and said her father made his own decisions.

A hearing on whether the case should be expedited is scheduled in Massachusetts on June 7 after Dauman filed a petition to have the trial date moved up.

Spokesmen for Sumner and Shari Redstone were not immediately available for comment on the letter from Salerno.

Shares of Viacom have fallen more than 50 percent in the past two years as its cable networks, including MTV and Nickelodeon, suffered from falling ratings because younger viewers were migrating online and to mobile video. Viacom's U.S. advertising revenue has declined for seven straight quarters.

Salerno said the directors were not happy with Viacom's current performance, and they were "very focused" on improving it and on "the need for management to drive it."

He added that they intend to continue with plans to "explore strategic options that might include a minority investment in Paramount," Viacom's movie studio.

Questions about Redstone's health have swirled since one of his former girlfriends, Manuela Herzer, filed a lawsuit last year arguing that he was not mentally competent to remove her from his advance healthcare directive.

The case, which claimed that Redstone was "a living ghost," was dismissed earlier this month. The mogul had struggled to speak when questioned by attorneys, but he was clear about wanting Herzer out of his life and putting his daughter in charge of healthcare decisions if he could no longer make them.

Since Redstone removed Dauman from the trust on May 20, Viacom shares have risen more than 13 percent, a move that some investors saw as the first step in a change in management that could eventually lead to a sale of the media company.