By Renee Maltezou and Jan Strupczewski ATHENS/BRUSSELS (Reuters) - Greek Prime Minister Alexis Tsipras called on his fractious Syriza party to rally behind the government on Tuesday as time runs critically short to unlock funding from international lenders and avert a debt default. Tsipras made his plea as Greek negotiators pushed to narrow differences with the European Union and IMF creditors over a cash-for-reform deal on which the country's future in the euro largely depends. With the Brussels negotiations entering a decisive phase, European officials took a tough stand, expressing their frustration with Greek negotiating tactics and trying to impress on Athens the depth of the risks that it is running.