Juba, South Sudan (CNN) -- A large group of youths from an ethnic militia loyal to former South Sudanese Vice President Riek Machar is marching for the city of Bor, just days after it was retaken by the government amid heavy fighting, officials said.
Col. Phillip Aguer, spokesman for the Sudan People's Liberation Army, estimated 20,000 ethnic Nuer from the so-called White Army were headed for the city, raising fears of more violence.
The U.N. Mission in South Sudan said Sunday it is "extremely concerned" about the report, but it could not confirm the size or location of the marchers.
"South Sudan does not need another escalation of the crisis involving armed youth, pitching communities against communities," said Hilde F. Johnson, the U.N. special representative of the secretary-general.
The U.N. mission warned that the involvement of armed youths "would add a volatile and unpredictable ingredient into the precarious security situation currently prevailing in the country."
The youths -- known for the white powder they use to cover their skin as an insect repellant -- do not have formal military training and, if not under direct orders, can act without discipline.
Information Minister Michael Makuei said Saturday that rebel leader Machar had gone to ethnic Nuer neighborhoods in Uror to mobilize the youths.
Multiple media outlets reported Machar denied the youths are acting under his orders. CNN has not been able to contact Machar's representatives.
The same group is believed responsible for the deaths of hundreds of civilians from the Murle ethnic group in 2011 and 2012.
Machar is a longtime rival of President Salva Kiir. The men are from two different tribal clans. Kiir is from the Dinka tribe, Machar from the Neur.
Kiir accused Machar of trying to stage a coup. Its' a claim Machar has denied.
Fighting broke out between Kiir's government and supporters of Machar on December 15 in the capital city of Juba. It quickly spread across the country, with reports of mass killings lent credence by mass graves.
Kiir's soldiers regained control of Bor last week in a bloody battle for the city.
A growing chorus of international agencies is calling for an end to the hostilities.
East African leaders on Friday gave the warring factions four days to lay down their arms.
If they don't, the leaders of the Intergovernmental Authority on Development warned they'll "take action" to stop the conflict that the United Nations says has killed more than 1,000 and forced some 121,000 from their homes.
The warning came the same day the United Nations said the first of 5,500 additional peacekeepers had arrived in the country.
The humanitarian crisis here is worsening, with at least 122,000 displaced since the fighting began. Roughly 63,000 have sought refuge in U.N. bases in the country, according to the world body.
Those on the U.N. base in Juba are scared to venture outside, even though many have homes within walking distance.
James Gajaak, the de facto community leader at the Juba U.N. camp, told CNN's Arwa Damon that when he first arrived with his family, he went three days without food and water.
"So my brother went back to the house to collect water," he said, recalling how later he received a phone call. "They tell me, 'Your brother is dead; come collect the dead body so that you can bury him.' "
Doctors from the relief group Doctors Without Borders are concerned over the increasing volume of people within the confined space of the camps.
Christine Bimansha is one of many doctors providing medical care at two UNMISS sites for Doctors Without Borders (Medecins Sans Frontieres). She said in an MSF field report Sunday that she is concerned about sanitation conditions and the potential spread of disease.
"Water and sanitation is the biggest problem the displaced people are facing -- they don't have access to water or to enough latrines. If the water and sanitation conditions at the UNMISS camp remain the same, the situation will get worse -- if people don't have water and we have an explosion of diseases like cholera, then that will be a big outbreak and a big problem. Measles is also a significant threat and an outbreak would be difficult to stop in such overcrowded conditions."
CNN's Antonia Mortensen reported from Juba, South Sudan. Neda Farshbaf reported from Atlanta. CNN's Sarah Aarthun contributed to this report.