(Updates with quotes from spokesman)
By Christine Murray and Tomás Sarmiento
MEXICO CITY, Sept 10 (Reuters) - Mexico's Grupo Salinas said on Wednesday its chairman will buy out its partner Grupo Televisa's 50 percent stake in the country's No. 3 wireless operator Iusacell for $717 million and seek a strategic partner to build up the carrier.
Televisa said separately it will make a book loss of around $320 million on the sale. It had already written down the bulk of what is a nearly $900 million loss on its original purchase.
The buyout, which is subject to regulatory approval, effectively reverses a deal approved two years ago after Televisa fought regulators for 14 months to be able to buy half the company for $1.6 billion.
Mexico's notoriously concentrated telecoms and broadcast sectors are in a period of flux after the government pushed through a major reform this year to try and foster competition.
Billionaire Carlos Slim's America Movil has said it would sell off some assets in order to reach below 50 percent market share in Mexican telecoms, after it was deemed a dominant market player.
Spain's Telefonica said in July it was in talks over a possible deal in Mexico, with sources saying it was targeting Iusacell.
Grupo Salinas spokesman Luis Nino de Rivera said the idea would be for billionaire owner Ricardo Salinas' future partner to take an equity stake in Iusacell.
"We are looking at all those interested in Mexico," Nino de Rivera told Reuters when asked if Telefonica was a candidate for strategic partner. He said around four or five companies were interested.
In recent months, Televisa has been turning its attention to the cable television market, where in August it paid $654 million for the rest of Cablecom.
"This is a pretty tough blow for Televisa," said Jose Otero, president of Signal Telecoms Consulting, saying that Televisa's remaining options to break into mobile were an alliance with the likes of Telefonica or starting a Mexican mobile virtual network operator (MVNO).
Salinas had said via Twitter in late July that the company's stake in Iusacell was not for sale.
Grupo Iusacell comprises two brands, Iusacell and Unefon. Some 25 percent of Iusacell's subscribers are post-paid subscriptions with higher income users, while Unefon serves the lower end of the market.
In contrast, around 14 percent of America Movil's subscribers in Mexico are post-paid. (Editing by Simon Gardner, Tom Brown and Bernard Orr)