By George Georgiopoulos and John O'Donnell ATHENS/FRANKFURT (Reuters) - Greece said it may impose capital controls and keep its banks shut on Monday after creditors refused to extend the country's bailout and savers queued to withdraw cash, taking Athens' standoff with the European Union and the International Monetary Fund to a dangerous new level. The European Central Bank said it would not raise the level of emergency funding, adding to the pressure on Greece's banks which have been surviving for the past few weeks on frequent incremental increases to the funding lifeline Amid political drama in Greece, where a clear majority wants to remain inside the euro, the next few days present a major challenge to the integrity of a 16-year-old currency bloc.