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A local radio station, Radio Tamazuj, reported fighting in the town and said witnesses confirmed that the rebels had withdrawn.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."
Members of the South Sudan rebel delegation attend the opening ceremony of South Sudan's peace negotiations, Addis Ababa, Ethiopia, January 4, 2014.
Taban Deng Gai, left, head of the rebel delegation and South Sudan's leader of the government delegation, Nhial Deng Nhial, attend the opening ceremony of South Sudan's peace negotiations, Addis Ababa, Ethiopia, January 4, 2014.
Unidentified members of the delegation from the South Sudan government and western observers meet at the Sheraton Hotel, Addis Ababa, Ethiopia, January 4, 2014.
A displaced mother and her baby, one of the few to have a mosquito net, wake up at a refugee camp, Awerial, South Sudan, Jan. 2, 2014.
A young displaced girl carries a bucket of water back to her makeshift shelter at a United Nations compound. The compound has become home to thousands of people displaced by the recent fighting, Juba, South Sudan, Dec. 31, 2013.
Displaced people gather inside a mosquito net tent as they flee from the fighting between the South Sudanese army and rebels in Bor town, in Juba, South Sudan, Dec. 30, 2013.
A displaced woman hangs up laundry on the plastic sheeting wall of a latrine at a United Nations compound, Juba, South Sudan, Dec. 31, 2013.
Yared, 2, is held by his mother, Madhn, who fled from the town of Bor a few days ago. She receives medicine for her child at a Medecins Sans Frontieres (MSF) medical tent, at a United Nations compound, Juba, South Sudan, Dec. 31, 2013.
A young displaced boy rests on the wheel arch of a water truck while others fill containers from it, at a United Nations compound, Juba, South Africa, Dec. 31, 2013.
A family makes tea outside their makeshift shelter at a United Nations compound, Juba, South Sudan, Dec. 31, 2013.
A general view of a camp for displaced people set up in a United Nations compound in Bor, South Sudan, Dec. 25, 2013.
South Sudan army soldiers hold their weapons as they ride on a truck in Bor, Dec. 25, 2013.