The United Nations says it is concerned by reports that thousands of armed youth loyal to former Vice President Riek Machar are preparing an attack on a South Sudan town.
A spokesman for the U.N. mission in South Sudan says a U.N. reconnaissance mission spotted a group of armed youths about 50 kilometers northeast of the central town of Bor. He says however that U.N. officials could not confirm how many people were in the group.
South Sudan's government claims that some 25,000 armed fighters from the Machar-backed force - the so called "White Army" - planned to attack the town, which was retaken by government forces this past week. The youth, like Machar, are ethnic Nuers while President Salva Kiir and his loyalists are ethnic Dinka.
Claims of the mobilization come as regional leaders attempt to broker a ceasefire.
The tribal violence erupted earlier this month, when the president accused Machar of attempting a coup.
The United Nations says the fighting has left more than 1,000 people dead and displaced tens of thousands.
A grouping of East Africa leaders announced Friday that South Sudan had agreed to a "cessation of hostilities" and the start of peace talks. The government also agreed to release eight of 11 political prisoners suspected of plotting the coup.
But Saturday, Machar ally Rebecca Nyandeng told VOA that Machar forces will not agree to cease-fire terms until the government releases all 11 prisoners.
Machar on Saturday stopped short of accepting the government offer. He told British radio the "mechanisms for monitoring" any agreement must first be established.
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.