WASHINGTON/BEIRUT, (Reuters) - President Barack Obama won the backing of key figures in the U.S. Congress, including Republi-cans, in his call for limited U.S. strikes on Syria to punish President Bashar al-Assad for his suspected use of chemical weapons against civilians. Speaking after the United Nations said two million Syrians had fled a conflict that posed the greatest threat to world peace since the Vietnam war, Obama said the United States also has a broader plan to help rebels defeat Assad’s forces. In remarks that appeared to question the legality of U.S. plans to strike Syria without U.N. backing, Secretary-General Ban Ki-moon said the use of force is only legal when it is in self-defence or with Security Council authorization. If U.N. inspectors confirm the use of chemical weapons in Syria, the Security Council, which has been deadlocked on the 2-1/2-year Syrian civil war, should overcome its differences and take action, Ban said.