By Michelle Martin BERLIN (Reuters) - German politicians kept up the pressure on Greece over the weekend to implement reforms, with Economy Minister Sigmar Gabriel warning Athens in an interview that a third aid package would not be on the cards unless the Greeks made some changes. Finance Minister Wolfgang Schaeuble suggested on Monday that Greece might need a referendum to approve painful economic reforms on which its creditors are insisting, and Gabriel said such a vote might speed up decisions. Athens has said it had no plans for a referendum at the moment. Gabriel, head of the Social Democrats (SPD), Chancellor Angela Merkel's junior coalition partner, stressed that the Greek government needed to take action in any case.