ATHENS (Reuters) - Greek Prime Minister Alexis Tsipras is due to meet senior advisers shortly to discuss whether to call early elections to quell a rebellion in his leftist Syriza party, a government official said on Thursday. Speculation of snap polls in September has grown in recent days as senior aides including Energy Minister Panos Skourletis openly call for a return to the ballot box. (Reporting by Renee Maltezou, Writing by Deepa Babington; editing by David Stamp)