By Julien Ponthus BRUSSELS (Reuters) - Euro zone leaders could hold a further emergency summit on Sunday to approve a plan to aid Greece if creditor institutions are satisfied with a Greek loan application and reform commitments, two senior EU sources said. The leaders were meeting on Tuesday evening in Brussels with Greek banks shuttered and fast running out of cash and the Greek economy on the verge of collapse. The sources set out a tentative timetable that could offer a breakthrough to keep Greece afloat and in the euro zone, provided it accepts credible reforms and spending cuts to make its public finances sustainable in the mid-term.