The United States has moved 150 U.S. Marines to Djibouti to respond, if required, to the crisis in South Sudan.
A spokesman for the U.S. military's Africa Command tells VOA the Marines would be used to help evacuate Americans and protect U.S. facilities in the troubled country.
Fighting between supporters of President Salva Kiir and his former vice president Riek Machar broke out on December 15. The U.N. says the fighting has displaced more than 80,000 people, amid reports of violence between the Dinka and Nuer ethnic groups.
On Tuesday, the U.N. human rights office said a mass grave has been discovered in the town of Bentiu, capital of Unity State. Spokeswoman Ravina Shamdasani tells VOA that a U.N. official saw 14 bodies in a grave and 20 at a nearby riverside.
She indicated the bodies could be those of some 75 Dinka army soldiers unaccounted for and feared dead.
The U.N. Security Council is due to vote Tuesday on a resolution to send 5,500 additional peacekeepers to South Sudan.
The U.S. says all members of the Council support the proposal. The troops would be transferred from other U.N. missions in Africa.
President Kiir said Monday in a meeting with U.S. special envoy Donald Booth that he is willing to hold talks with Machar without preconditions.
The country's Foreign Minister Barnaba Marial Benjamin told VOA that Mr. Kiir stressed in remarks to parliament that talks are the way to end the violence.
"President Salva Kiir, being an elected president democratically, is responsible for the lives of the people of South Sudan, including the foreigners in this country, so it is his absolute constitutional right and mandate to see that peace is achieved. And I hope Dr. Riek Machar should also be able to see the same, that the people of this country suffered so much and cannot be losing their lives because of a power struggle."
On Tuesday, the White House released Dinka- and Nuer-language versions of a recent statement by President Barack Obama, appealing for an end to the violence.
Mr. Kiir, an ethnic Dinka, blames Machar, an ethic Nuer, for what he says was an attempted military coup that triggered the violence. Hundreds of people have died in the fighting.
Machar told Reuters on Monday that he will take part in dialogue immediately if Mr. Kiir releases detained opposition leaders.