** Shares of Chilean telecommunications firm Entel ENTEL.SN rise 5% on potential joint bid for Telefonica Chile assets
** America Movil AMXB.MX says it signed non-binding agreement with Entel to explore joint offer for Telefonica's operations in Chile
** Agreement allows either company to opt out or submit individual offers
** Entel says due diligence ongoing, no decision yet on a binding offer, Chilean market regulator says
** The stock pares gains after climbing 7.6%, hitting its highest level since January 2013
** Meanwhile, shares of America Movil rise slightly by 0.2% on Mexico's main stock index
(Reporting by Natalia Siniawski; Editing by Aida Pelaez-Fernandez)
((Natalia.Siniawski@thomsonreuters.com;))