Three U.S. planes trying to evacuate refugees from South Sudan to Uganda were fired on Saturday, injuring four U.S. service members, the U.S. military says. One of the U.S. service members was wounded seriously. The New York Times said the aircraft were forced to return to Uganda without completing their mission. U.S. President Obama said Thursday he was sending 45 U.S. service members to South Sudan to "support the security of U.S. personnel and our embassy." Previous evacuation flights had been organized in Juba, the South Sudanese capital, but the mission Saturday apparently was the first into rebel-held territory, the Times said. The three aircraft were CV-22 Ospreys, tilt-rotor craft that can fly like a plane and land like a helicopter, to evacuate U.S. citizens from a U.N. compound in Bor, the capital of Jonglei State, the report said. The Times said the seriously wounded U.S. service member was flown to Nairobi, Kenya, where he was in surgery Saturday night. Bor is occupied by forces loyal to former vice president Riek Machar, who was dismissed from office in July, the BBC said. President Salva Kiir accused him last week of attempting a coup. Some 500 people have been killed in fighting, prompting U.S. Secretary of State John Kerry to send a special envoy to the region Friday, CNN reported. In a statement, Kerry called for South Sudan leaders to rein in armed groups under their control, cease attacks on civilians, "and end the chain of retributive violence between different ethnic and political groups." "The violence must stop, the dialogue must intensify," he added. National security adviser Susan Rice, in a recorded message, urged the people of South Sudan to "make the choice for a unified and cohesive South Sudan" for "yourselves and your children." The United Nations said Friday two Indian peacekeepers were killed and a third wounded when gunmen attacked a U.N. building Thursday. Up to 20 of 30 civilians who had taken refuge in the building may have died.