MILAN/MADRID (Reuters) - Italy's Atlantia (>> Atlantia) plans to improve its offer for Spain's Abertis (>> Abertis Infraestructuras) to trump a 17.1 billion euros (£15.22 billion) rival bid from ACS' German subsidiary Hochtief (>> Hochtief), two sources close to the matter said on Thursday.

Infrastructure group Atlantia is studying a rejigged cash-and-share bid but wants to wait for Spain's market watchdog to first approve Hochtief's offer before making a move, the sources said.

ACS-Hochtief unveiled their proposal last week but they are still looking for investors who want to team up to acquire Abertis, one of the sources said.

They are also considering selling some Abertis assets, including a minority stake in French motorway group Sanef, should their bid on the Spanish toll-road operator be successful, three sources said.

Atlantia, Hochtief and Sanef all declined to comment on the issues.

(Reporting by Francesca Landini and Paola Arosio in Milan, Andres Gonzalez in Madrid, Matthias Inverardi in Duesseldorf, Dominque Vidalon in Paris, editing by Valentina Za)