PHNOM PENH, Cambodia—U.S. President Barack Obama supported Southeast Asian nations' proposed multilateral approach to tackling territorial tension in the South China Sea, pushing back against Beijing as a major international meeting became bogged down in acrimony for the second time in five months.

Mr. Obama met China's outgoing premier, Wen Jiabao, at the East Asian Summit on Tuesday, the final day of a tour that brought him to Thailand and Myanmar. The pair made pro-forma comments...