The State Department has classified these 11 nations as "Do not travel."
The U.S. backed the country in its independence struggle. Now, it is needed again.
A U.S. intelligence agent is assisting with the prosecution of suspects accused of killing a South Sudanese aid worker and raping his foreign colleagues.
Soldiers killed 114 civilians, raped countless women and attacked funerals in a recent offensive, according to a U.N. report.
Almost half the population face food shortages and 100,000 people are at risk of immediate starvation in South Sudan.
At least 82 aid workers have been killed since 2013 in South Sudan, the U.N. says.
Some 16 million people in the region are in urgent need of food, water and healthcare.
Nearly half the population will not have a reliable food source by summer.
The U.N. has warned of possible genocide in South Sudan.
Western countries should follow the example of Uganda, an aid agency has said.
The Security Council has failed to impose an arms embargo on the conflict-hit state.
Political change, separatist insurgencies and a possible genocide project a tough year for the continent.
"If we fail to act, South Sudan will be on a trajectory toward mass atrocities," Ban told the Security Council.
A U.N. human rights commission said that ethnic cleansing was already underway in South Sudan.
Russia said U.N. mission in South Sudan is “in ruins” after damning inquiry on peacekeepers.