By Renee Maltezou and Angeliki Koutantou ATHENS (Reuters) - Greek Prime Minister Alexis Tsipras, struggling to contain a revolt in his left-wing Syriza party, said on Wednesday that his government would not implement reform measures beyond those agreed with lenders at a euro zone summit this month. Tsipras faces a tough Syriza central committee session on Thursday with many activists angered by his acceptance of bailout terms more stringent than those voters rejected in a July 5 referendum. In a clear warning to party rebels, Tsipras said he could be forced to call early elections if he no longer had a parliamentary majority, and suggested an emergency party congress could be held in early September.