From the U.S. State Department:
The U.S. Department of State continues to warn U.S. citizens of the risks of travel to the Republic of South Sudan and strongly recommends U.S. citizens defer all travel to the country. This replaces the Travel Warning dated July 12, 2011, updates information on security incidents, and reminds U.S. citizens of ongoing security concerns in the Republic of South Sudan.
The Department of State warns U.S. citizens of the risks of traveling to South Sudan and strongly recommends that you avoid all travel to the states in the border region between Sudan and South Sudan (Upper Nile, Unity, and Western Bar el Ghazai states in South Sudan; Southern Kordofan and Blue Nile states in Sudan; and the Abyei Special Administrative District). Although fighting between Sudan Armed Forces (SAF) and the Sudan People’s Liberation Army (SPLA) has declined since spring 2012, the potential for troop build-ups along the border and renewed fighting continues to be a legitimate threat.
You should exercise extreme caution in all areas of South Sudan. In addition to the fighting in the border region, there are at least seven rebel militia forces that frequently engage in violent clashes with SPLA forces in various areas of South Sudan; these clashes can flare up with little warning and may exacerbate ethnic tensions throughout the country, leading to further violence.
The Government of South Sudan has limited capacity to deter crime or provide security to travelers outside of the capital city of Juba.
U.S. State Department Warning