Europe's debt drama dominated talks among leaders of the Group of 20 leading economies Thursday, with Greece's government facing potential collapse and European leaders admitting that the eurozone may face losing its weakest member. U.S. President Barack Obama, Chinese President Hu Jintao and other world leaders began the formal G-20 summit after a morning packed with bilateral meetings and a hurriedly organized meeting of eurozone officials to respond to developments in Greece which risk fracturing Europe's common currency area and the EU itself. President Barack Obama said the most important task at the G-20 summit is to resolve the European financial crisis and urged European Union leaders to flesh out details of their ambitious plan to rescue Greece and stabilize financial markets. French President Nicolas Sarkozy scrambled to save the summit he's hosting from being hijacked by the tumult in Greece. Sarkozy said he and Obama agree the private sector should play a greater role in helping resolve the global financial crisis.