The U.S. military says unidentified gunmen have opened fire on U.S. aircraft flying in to a rebel-held area of South Sudan to evacuate American citizens. Four U.S. service personnel were reported wounded.
The incident took place Saturday as U.S. forces were trying to rescue Americans from the heavily-contested town of Bor, north of the capital, Juba. The mission was aborted and all three planes were diverted to neighboring Uganda.
U.S. President Barack Obama sent 45 U.S. service personnel to the region Thursday on a mission he said is aimed at protecting U.S. personnel and the U.S. embassy.
In a White House statement Saturday, Mr. Obama stressed the importance of the U.S. evacuation mission and said South Sudan's leaders have a responsibility to assist the U.S. efforts.
South Sudan's government says rebels have overrun Bor - the scene of some of the country's worst fighting over the past week.
U.N. Secretary-General Ban Ki-moon called on the South Sudan leaders Sunday to find a "political means" to address the conflict. He said the continued violence poses a "dangerous threat" to the future of the young country.
The violence erupted after President Salva Kiir, a member of the Dinka tribe, accused former vice president Riek Machar, a Nuer, of attempting a coup. The Juba government says more than 500 people have been killed, and the unrest has forced tens of thousands of people to flee from their homes.
VOA's Hanna McNeish describes the scene in Juba.
"There is still a 6 p.m. to 6 a.m. curfew in place and there are military tanks and 'technicals' [armored vehicles] and a lot of men with guns sitting on every street corner. But it is nothing compared to the situation outside the capital. We've got fighting in Jonglei state, where rebels have taken over the state capital, Bor. We've also got heavy fighting, apparently, in Unity state, where the oil fields and rebels are battling government forces for control of these."
McNeish says many people that she has spoken to fear that what started as a political power struggle is turning into a deeply ethnic conflict. She says one of the most stunning scenes in Juba is the capital's crowded airport.
"Seeing hundreds of people trying to leave by charter flight or by any means possible, even with a single bag, because they are so worried that this violence has just spiraled out of control already and they don't trust that the government of South Sudan can protect them any longer."
U.S. Secretary of State John Kerry says he is sending a special envoy to South Sudan to encourage talks between opposing factions. Ambassador Donald Booth is heading to the region soon.
The U.N. Security Council president said Friday that South Sudan's President Kiir and former vice president Machar have agreed to "unconditional dialogue" despite their recent history of bitter recriminations. Mediators from East African countries met with Mr. Kiir Friday in what they called productive talks, but what form the Kiir-Machar "dialogue" will take is not clear.
On Friday, the U.N. said at least 11 civilians and two peacekeepers had been killed in an attack a day earlier on a U.N. base in the town of Akobo.
The U.N. Mission in South Sudan (UNMISS) says the peacekeepers and civilians were shot after about 2,000 armed youth, believed to be ethnic Nuers, surrounded the base and opened fire on members of the Dinka ethnic group taking shelter at the U.N. compound.
About 35,000 civilians are believed to have fled to U.N. compounds since the latest unrest began.
South Sudan is is the world's newest country, gaining its independence from Sudan in 2011.