UNITED NATIONS (Reuters) - U.N. Secretary-General Ban Ki-moon said on Tuesday that the use of force is only legal when it is in self-defense or with Security Council authorization, remarks that appear to question the legality of U.S. plans to strike Syria without U.N. backing. He also suggested that a U.S. attack could lead to further turmoil in conflict-ravaged Syria, where the United Nations says over 100,000 people have been killed in the country's 2-1/2-year civil war. Ban was speaking to reporters after President Barack Obama won the backing of two top Republicans in Congress in his call for limited...