The United Nations fears that armed men could overwealm their forces at small UN bases in South Sudan. The UN has already come under attack in Akobo. Its base there was overrun on Friday by two thousand youths from the Nuer ethnic group. In other developments, two U.S. military aircraft have been shot at during an evacutation mission - three U.S. service personnel were injured. Al Jazeera's Jamal Elshayal reports.