Story highlights Obama warns further U.S. action is possible

"U.S. citizens and citizens from our partner nations" were flown out, official says

U.N. says 62,000 people displaced by violence

South Sudan violence has spread from capital Juba farther north in one week

All Americans who presented themselves at the United Nations camp in Bor, South Sudan, were evacuated safely Sunday amid deadly violence in the country, the State Department said. A State Department official said about 15 Americans were flown out.

U.S. personnel are working to confirm that no other U.S. citizens remain in Bor in need of evacuation, a State Department official said.

"This morning, the United States -- in coordination with the United Nations and in consultation with the South Sudanese government -- safely evacuated American citizens from Bor, South Sudan," State Department spokeswoman Jen Psaki said in a statement. "U.S. citizens and citizens from our partner nations were flown from Bor to Juba on U.N. and U.S. civilian helicopters. The United States and the United Nations, which has the lead for securing Bor airport in South Sudan, took steps to ensure fighting factions were aware these flights were a humanitarian mission."

"The U.S. government is doing everything possible to ensure the safety and security of United States citizens in South Sudan. We are working with our allies around the world to connect with and evacuate U.S. citizens as quickly and safely as possible," Psaki said.

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The United Nations moved noncritical staff out of South Sudan's capital, Juba, across the border into Uganda on Sunday, as the violence spread inside the world's newest country.

U.N. civilian staff were moved from a compound in the flashpoint town of Bor to Juba on Saturday, the same day a U.S. mission to airlift Americans out was aborted when the aircraft came under fire.

U.S. President Barack Obama said in a letter to congressional leaders Sunday that 46 U.S. service members took part in the mission, and he noted he "may take further action to support the security of U.S. citizens, personnel and property, including our Embassy, in South Sudan."

Four U.S. troops were wounded in the attack in Bor and were expected to be moved to the U.S. military hospital at Landstuhl, Germany, a senior U.S. official told CNN earlier Sunday.

One of the injured "went through some pretty serious surgery" after being taken to Nairobi, Kenya, for wounds from the gunshots fired at the aircraft. All four have been able to speak to their families.

Thousands displaced

Hundreds of people have been killed in a week of fighting, which has spread from Juba to oil fields farther north.

South Sudanese President Salva Kiir blamed soldiers loyal to his former vice president, Riek Machar, for starting the violence.

Up to 40,000 civilians have taken refuge in United Nations bases in the country, Secretary-General Ban Ki-moon said Sunday.

"There are many more thousands of people who are very much in fear and vulnerable, and at this time, the priority of the United Nations is to (protect) the lives of civilians," Ban told a news conference in the Philippines.

The U.N. Office for the Coordination of Humanitarian Affairs estimates some 62,000 people have been displaced in total, with five of South Sudan's ten states affected by the violence.

The U.N. has said some 20 people were killed during an attack Thursday by about 2,000 armed youths on a U.N. peacekeeping base in Jonglei state. Two Indian peacekeepers were also killed.

After the attack, the assailants fled with arms, ammunition and other supplies, the U.N. said.

'We will not be intimidated'

In a statement Sunday, the U.N. said it had made the decision to relocate noncritical staff from Juba to Entebbe, Uganda, as "a precautionary measure to reduce pressures on its limited resources," while still providing assistance and shelter to more than 20,000 civilians who have gathered at its compounds in the capital.

The U.N. Mission in South Sudan is planning to reinforce its military presence in Bor and Pariang, the organization said. Plans are also under way to relocate all noncritical staff from the U.N. compound near Bentiu in Unity state, and reinforcements will also be sent in.

"We are not abandoning South Sudan. We are here to stay, and will carry on in our collective resolve to work with and for the people of South Sudan," U.N. envoy Hilde F. Johnson said in the statement. "To anyone who wants to threaten us, attack us or put obstacles in our way, our message remains loud and clear: we will not be intimidated."

Ban said the U.N. would "transfer our assets from other peacekeeping missions," such as the one in the Democratic Republic of Congo and other areas, to the troubled country.

"We also seeking support from other key countries who can provide the necessary assets," he said. "We are in shortage of capacity. When the United Nations compound was overrun by 2,000 armed elements, we were having difficulties."

Echoing calls from Western governments, Ban urged an end to the violence and called on Kiir and Machar "to come to the table and find a political way out of this crisis."

"They are responsible to the people of South Sudan to end the crisis and find the political means of addressing their differences," he said.

Ban said he had dispatched a special representative to Juba to work with the U.N. envoy in the city. Washington is also sending an envoy.

Tensions have been high in South Sudan since July, when Kiir dismissed Machar and the rest of the Cabinet. The move inflamed tensions between Kiir's Dinka community and Machar's Nuer community.

South Sudan formally split from Sudan in 2011 after a referendum, following decades of conflict. Numerous armed groups remain active in the oil-rich country.