The US military has said that 150 marines were ready to enter South Sudan, the world's newest state, to help evacuate Americans and protect the US embassy in the face of a looming civil war.
American roops have been positioned in neighbouring Djibouti, the site of the only US military base in Africa.
"By positioning these forces forward, we are able to more quickly respond to crisis in the region, if required," said the Africa Command.
"One of the lessons learned from the tragic events in Benghazi [Libya] was that we needed to be better postured, in order to respond to developing or crisis situations, if needed. These precautionary movements will allow us to do just that," it said.
South Sudan has been reeling under intensified violence in the past few days as rival army factions launch attacks against each other. Officials have been expressing concern that the situation could easily spill over into a bloody ethnic conflict.
As feared earlier, government forces backing President Salva Kiir, who belongs to the Dinka tribe, shot dead dozens of people on the outskirts of the capital Juba, mostly Nuer neighbourhoods, according to local reports.
A mass grave containing 75 bodies has been found in South Sudan's Unity state, according to the UN.
UN High commissioner for human rights Navi Pillay appealed on both sides to protect civilians.
"Mass extrajudicial killings, the targeting of individuals on the basis of their ethnicity and arbitrary detentions have been documented in recent days," Pillay said in a statement.
"We have discovered a mass grave in Bentiu, in Unity state, and there are reportedly at least two other mass graves in Juba."
The UN chief Ban Ki-moon has called for a sharp increase in the international troop presence in order to quell the spiralling violence.
In a letter to the 15-member Security Council, he called for 5,500 more peacekeepers in South Sudan. The landlocked middle African country currently has about 6,700 UN troops and 400 police personnel.