ATHENS (Reuters) - Greece's Socialist party PASOK called for the resignation of Prime Minister Alexis Tsipras on Saturday after he called a snap referendum on international creditors' demands in a cash-for-reforms deal. "Since Mr. Tsipras is unable to take responsible decisions, he ought to resign and let citizens vote for their future via elections," PASOK's leader Fofi Genimmata said in a statement. (Reporting By Michele Kambas)