South Sudan's government says its army has retaken one of two state capitals seized by renegade soldiers last week.
The government said in a Twitter message that its forces have retaken Bor, the capital of Jonglei state, and are clearing out rebel forces.
A local radio station, Radio Tamazuj, reported fighting in the town and said witnesses confirmed that the rebels had withdrawn.
The United Nations Security Council voted Tuesday on a resolution to send 5,500 additional peacekeepers to South Sudan, boosting its force to 12,500.
Fighting that followed an alleged coup attempt last week has left hundreds dead and displaced more than 80,000 people, amid reports of violence between members of the Dinka and Nuer ethnic groups.
The United States says 150 Marines have been moved to Djibouti, ready to enter South Sudan to evacuate Americans and protect U.S. facilities.
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 told 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.
Bentiu is the other capital seized by soldiers believed to be loyal to former vice president Riek Machar.
President Salva Kiir, an ethnic Dinka, blames Machar, an ethnic Nuer, of masterminding the alleged coup attempt December 15.
Kiir told U.S. special envoy Donald Booth on Monday that he is willing to hold talks with Machar without preconditions.
The country's foreign minister, Barnaba Marial Benjamin, told VOA that 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," said Benjamin.
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.
Machar told Reuters on Monday that he will take part in dialogue immediately if Kiir releases detained opposition leaders.
The government said Tuesday it "will not release anybody accused of a coup."